Heat the oil in a large skillet or pot over medium-high heat.
Add the stew meat and cook until browned on all sides.
Add the diced onion and bell pepper (if using) to the pot and cook until the onion is translucent.
Stir in the tomato sauce (or diced tomato), garlic powder (or minced garlic), cumin, salt, and black pepper.
Sprinkle the flour over the mixture and stir well to combine.
Cook for an additional 2 minutes, stirring constantly.
Gradually pour in the water (or chicken stock), stirring well to remove any lumps.
Bring the mixture to a boil, then reduce the heat to low and cover the pot.
Simmer for 1 to 2 hours, or until the meat is tender and the flavors are well combined.
Serve the carne guisada with warm flour tortillas, rice, or mashed potatoes.