Note: The ricotta should be well-drained, almost dry (or else it will take longer to cook down). If it is not, drain it in a fine mesh strainer lined with a piece of cheesecloth or a coffee filter to get rid of extra moisture, then spread it out to dry a little before using it.
When you're ready to cook, heat the oil in a frying pan, add the onion and fresh chiles, and fry gently without browning for 1 minute.
Add the tomatoes and continue cooking over high heat, stirring from time to time until the mixture is fairly dry -- about 4 minutes.
Add the ricotta and salt and mix well. Cook over medium heat until it begins to turn a light creamy color and is no longer liquid: when turned with a spoon it should come away cleanly from the surface of the pan, about 15 to 20 minutes, depending on how well you've drained your ricotta. Serve immediately with corn tortillas.