Char the tomatoes, half onion, serranos and unpeeled garlic in a dry pan over high heat, turning, until blackened in patches, about 8 minutes. Peel the garlic.

Blend them with 1 teaspoon of the salt into a slightly coarse sauce.

Heat 1 tablespoon of the oil in a pan and pour in the sauce. Fry it, stirring, for 5 minutes, until it darkens and thickens. Keep it warm.

In a separate pan, fry each tortilla in a little of the remaining oil for about 30 seconds a side, until it is firm at the edges but still flexible in the middle. Drain on a rack.

Fry the eggs in the same pan over medium-low heat, covered, 3 minutes, until the whites are set and the yolks still liquid.

{'Build each plate immediately': 'two tortillas, a spoon of warm mashed beans, two eggs, then the warm sauce ladled around the yolks rather than over them.'}

Scatter with queso fresco, cilantro and diced raw onion, and serve at once.