Heat the oil in a large skillet (cast iron, if you have it) over high heat. Add the meat and brown for 10 minutes. Do not crowd the pan or the meat with steam, so to cook all the meat may take two batches.
Do not drain the liquid or remove the meat, but add the bell peppers, onions, garlic, serrano chile and tomatoes and continue cooking for 10 minutes.
Add the spices, tomato sauce and 2 cups of water. Reduce the heat to low and simmer covered for 30 minutes.
After 30 minutes, remove the lid, stir well and cooking another 30 minutes uncovered. Now the sauce will begin to evaporate and thicken. Add more water as required and season to taste. The meat should be very tender.
In a small cup or bowl, combine the flour with 3 tablespoons cooking liquid. Stir well to remove any lumps and then add to the carne guisada, mixing well until evenly distributed.
Cook another 3-5 minutes to "cook" the flour taste out and serve.