In a skillet over medium heat, combine the cooked ground beef, taco seasoning, diced tomatoes, and diced onions. Cook for about 5 minutes until heated through and the flavors meld.

Lay the flour tortilla flat on a clean surface. In the center, layer the beef mixture, cheddar cheese, and mozzarella cheese.

Fold the sides of the tortilla inwards, then roll it up tightly from the bottom to form a burrito.

In the same skillet, melt the butter over medium heat. Place the burrito seam-side down in the skillet and cook for about 3-4 minutes until golden brown.

Carefully flip the burrito and cook for an additional 3-4 minutes on the other side until crispy and the cheese is melted.

Remove from the skillet, slice in half, and serve with salsa on the side.