Preheat the oven to 200 degrees.
Heat the olive oil in a large skillet over medium high heat. Add the beef, spreading it out into smaller chunks across the pan. Cook for 2-3 minutes or until browned on the bottom. Flip, breaking up into smaller pieces, then add the onions and peppers and continue to saute another 3-4 minutes, or until beef is cooked through and veggies are softened. Drain any excess grease off the pan.
Sprinkle evenly with taco seasoning, then add the water and tomato sauce. Stir until well combined and just slightly thickened. Remove the beef mixture to a plate.
Wipe out the skillet and place over medium heat. Melt a dollop of butter in the pan, swirling it so it covers the edges. Place a tortilla in the pan, then sprinkle evenly with about 1 cup of cheese. Heat until the cheese is just starting to melt, about 2 minutes.
Add a quarter of the filling mixture to one half of the tortilla, then use a spatula to lift the cheesy side over top of the beef side. Use a spatula to press down, so the cheese melts into the beef on both sides. Cook another minute or two, until the bottom side is golden, then flip and cook the other side another 1-2 minutes if needed.
Remove from the skillet and place on a wire rack in a 200 degree oven to keep warm. Repeat with remaining quesadillas.