For a smoky, flame-grilled flavour, grill the burger patties over an open fire. Lightly toast the inside of the rolls to prevent the bread from getting soggy.
Using your hands, combine the minced beef with the red kidney beans, diced red onion, garlic, cayenne pepper and egg and generously season with salt and pepper.
Divide the mince into 4 parts and roll each piece into a ball, then form into flat patties roughly 10cm wide and about 2cm thick.
To help firm up the patties, drizzle with olive oil, put them on a plate, cover and place in the fridge until needed.
To make the salsa roja sauce, heat a few tablespoons of olive oil in a large frying pan over a medium-high heat and cook the onions and tomatoes until they are nicely charred. This will add rich flavour to the sauce so don’t worry if the edges are a bit blackened.
In a separate pan, dry fry the garlic, chillies and cumin for 2 minutes until fragrant.
Add the chilli mixture to the tomatoes, turn the heat to low and simmer for about 15 minutes.
Make the guacamole whilst the tomatoes are simmering. Halve and pit the avocados and using a tablespoon scoop out their flesh and roughly mash with a fork, leaving them a bit chunky. Mix in the lime juice, cumin and tasting the guacamole, season with salt and pepper to your liking.
Heat a griddle pan on a medium-high heat, brush the burger patties with oil and cook for 3 or 4 minutes on each side or to your liking. Use a spatula to lightly press down on them, making sure each burger is in full contact with the pan.
Finish the salsa roja whilst the burgers are cooking by blending the tomato mixture and the fresh coriander sprigs in a food processor/blender and puree until smooth. Stir in the lime juice, oreganum and sugar and season with salt and pepper to taste. Serve warm.
Build your burger by topping it with lettuce leaves, slices of fresh tomato, guacamole, grated cheese and a couple spoonfuls of the spicy salsa roja. ¡Delicioso!