Mexican magic: Santiago Lastra’s recipes for cheesy mushroom costras and beetroot tostadas

Costra, meaning ‘crust’ in Spanish, is a classic dish from the north and centre of Mexico that’s traditionally made with tender cuts of beef and finished with a melted crust of semi-hard cheese (usually Chihuahua or manchego mixed with Oaxaca cheese for texture). The cheese is grated on a plancha grill and allowed to melt and crisp up, and is then draped over the meat. Here, though, I have substituted the steak for mushrooms. Then, tostadas are traditionally made with crisp corn tortillas that are either baked or deep-fried, a technique that was first invented to preserve the tortillas for longer. They’re commonly used to serve lean, light preparations, like ceviches, aguachiles and salsas. Here, I’ve used poppadoms to achieve a similar texture and starchiness; use ready-cooked ones to make this even quicker.

Beetroot tostadas (pictured top)

Prep 10 min
Cook 2 hr
Makes 10

For the sesame macha (makes 750ml)
2 garlic cloves
120g sesame seeds
500ml vegetable oil
4 tbsp chilli flakes
2 tsp smoked paprika
2½ tbsp honey
2 tsp flaky sea salt

For the pink mole (makes 250g)
45g pine nuts
60g beetroot, unpeeled
3 garlic cloves
1½ tsp smoked paprika
2 dried bird’s eye chillies (or to taste)

½ tsp flaky sea salt

For the tostadas
3 mixed colour beetroots, unpeeled
Vegetable oil, for drizzling and deep-frying
1 tsp flaky sea salt
10 uncooked poppadoms, or use ready-cooked ones
1 ball burrata
1 bunch soft herbs of your choice, such as oregano, parsley, chervil, mint, basil
½ quantity pink mole (see above and method)
½ quantity sesame macha (see above and method)

Heat the oven to 210C (190C fan)/400F/gas 6½. Toast the garlic in a dry pan, moving it constantly, for eight to 10 minutes, until the skins are charred and the insides creamy. Put the sesame seeds on a baking sheet, toast in the oven for five minutes, until golden, then remove and put 20g of the toasted seeds in a bowl.

Pour enough oil into a frying pan to just coat the bottom, and put it on a medium heat. Add the chilli flakes, toast for about a minute; careful they don’t burn, then pour into a food processor. Add the remaining 100g toasted sesame seeds and all the other macha ingredients and blend until smooth. Store at room temperature for up to a week. When ready to serve, finish with the reserved sesame seeds.

For the pink mole, toast the pine nuts on a baking tray in the oven for around eight minutes, giving the tray a shake halfway through, then set aside. Turn up the oven to 220C (200C fan)/425F/gas 7. Wrap the 60g beetroot in foil then put the three other beetroot on a sheet of foil, drizzle with oil, and roast them all for an hour and 30 minutes until soft. Once cool enough to handle, peel (use gloves to avoid staining your hands).

Toast the unpeeled garlic in a dry frying pan over a medium heat for about 10 minutes, until golden, then squeeze out the flesh. Put the beetroot for the pink mole, pine nuts and garlic in a food processor with all the remaining mole ingredients. Add 50ml water to begin with and blend until smooth, adding more water, if need be. To control the spice level, you can add the chillies one by one. Store in the fridge (it will last for three to four days) and stir before using.

Put the three peeled beetroot in a mortar or bowl and roughly mash with a pestle, potato masher or fork. Season with a little salt.

Half-fill a saucepan with oil and heat to 180C. Deep-fry the poppadoms for three minutes or until crisp and puffed up, then remove from the oil with a slotted spoon and drain on paper towels.

Put the burrata on a plate and dress with the herbs. Sprinkle over a pinch of salt and drizzle with a little oil. Put the pink mole and sesame macha into small bowls, then build your tostadas at the table.

Mushroom costra

Santiago Lastra’s mushroom costra.

Prep 20 min
Cook 50 min
Serves 6

Put all the ingredients for the salsa verde in a food processor, add 25ml water and blend, adding up to 25ml more water as needed, until the mixture is smooth and homogenous. If you would like to control the spiciness, add the chillies one by one until the mix is at your desired level of heat. Store in the fridge and stir before serving. The salsa is best served fresh, but it will keep refrigerated for one to two days.

Heat the oven to 220C (200C fan)/425F/gas 7. Put the mushrooms in a baking tray, drizzle with oil and season with salt, then roast for 20 minutes. Remove from the oven and, while the mushrooms are still hot, flatten them with a rolling pin, rolling them out much as you would roll out dough.

Put a dry frying pan on a medium heat and cook the tortillas for 30 seconds on each side until slightly bubbling – be careful not to dry them out. Wrap them in a cloth to keep warm.

Divide the cheese into 25g balls. Put one ball in the same pan and spread it out to create a circle about the same size as one of the mushrooms. Cook gently until slightly melted, then put a mushroom on top. Once the cheese has caramelised a little – about five minutes over a medium heat – and is golden on the bottom, lift the cheesy mushroom out of the pan and set aside. Repeat with the remaining cheese and mushrooms.

Put the cheesy mushrooms on the hot tortillas and finish with the coriander, onion, salsa and a squeeze of lemon.

• These recipes are edited extracts from Mexican Soul: A New Style of Cooking, by Santiago Lastra, published by Quadrille at £30 in August.