To make the tomato sauce: Heat the olive oil over medium heat in a small saucepan. Add the onion and garlic and cook, stirring, until the onion is translucent, about 5 minutes. Stir in the chili powder, cumin, and oregano. Cook, stirring, until fragrant, 1 to 2 minutes. Stir in the tomato sauce and season, to taste, with salt and pepper. Stir in the stock, bring to a boil, then adjust the heat so the sauce is simmering. Cook, stirring occasionally, until the sauce is lightly thickened, about 15 minutes.
Set the rack about 4 inches from the broiler and preheat the broiler.
Cook the chorizo in a medium skillet over medium heat, stirring occasionally, until fragrant, 3 to 4 minutes. Drain on paper towels and set aside
Cut the avocados in half, remove the pit and peel the halves. Cut each half lengthwise into 6 thin slices. Toss together in a medium bowl with the lime juice. Divide the chorizo among 6 oven-proof, individual serving casseroles, about 5 1/ 2-inches wide by 1-inch deep. Cover the chorizo in each casserole with 4 thin slices of avocado.
To cook the eggs: Spray a frying pan or griddle with the vegetable cooking spray and heat over medium heat. Add as many eggs as will fit without touching and cook until the underside of the eggs is set, about 1 minute. Reduce the heat to low and cook until the whites are completely set, about 3 minutes. Place 2 eggs on top of the avocado in each casserole. Repeat with the remaining eggs.
Divide the tomato sauce among the 6 casseroles, topping the eggs with a thin, even layer of sauce. Sprinkle the cheese over the sauce, dividing it evenly. Broil just until the cheese melts, about 1 minute. Top each casserole with a teaspoon of crema and sprinkle the chopped cilantro over the cream. Pass the remaining crema separately.
Cook's Notes: Crema Mexicana is a mildly acidic dairy product with a consistency that makes it perfect for drizzling over finished dishes. It is available in Latin markets and some supermarkets. If you cannot find it, substitute sour cream, thinned down with water.
Oaxaca cheese is a mild string cheese from the state of Oaxaca in Mexico. It is available in Latin markets and some supermarkets. Whole milk mozzarella is the perfect substitute.