Prepare the ingredients. Thinly slice the onion. Heat the olive oil in a pan over medium heat and sauté the onion until it softens but does not brown.

Add the tomato puree and oregano. Cook until the color deepens, about 10 minutes, stirring occasionally to prevent sticking. I prefer natural puree because the seasoned kind contains starch and other additives I don't like. Season with salt to taste. Spread a little of the sauce on a plate. Warm the tortillas slightly so they don't get hard, fill each with ricotta cheese, and fold in half.

Top with the tomato sauce, then add lettuce, sour cream, and Cotija cheese.