01 - Heat olive oil in a skillet over medium heat. Add onions and cook until softened, about 2 minutes. Add garlic and sauté for 30 seconds.
02 - Add ground beef and cook, breaking it up, until browned, about 5 minutes. Stir in taco seasoning, salt, pepper, tomato paste, and water. Simmer for 2–3 minutes until thickened. Remove from heat.
03 - Warm tortillas in a dry skillet or microwave until pliable.
04 - Lay each tortilla flat. In the center, layer 1/4 of the rice, beef mixture, black beans, tomato, salsa, sour cream, and a mix of both cheeses. Sprinkle with cilantro if using. Fold in the sides and roll up tightly to form burritos.
05 - Spread a thin layer of butter on the outside of each burrito. Heat a large skillet or griddle over medium heat. Place burritos seam-side down and grill for 2–3 minutes per side, pressing gently, until the tortillas are golden and the cheese is melted.
06 - Slice in half and serve hot.