Preheat oven to 375°F.
In a skillet over medium heat, cook the ground beef and onions until browned.
Stir in taco seasoning, tomato sauce, and cream cheese. Mix until creamy and well blended.
Remove from heat. Stir in 1/2 cup cheddar cheese until melted.
Spoon the mixture into each tortilla, roll them up, and place seam-side down in a baking dish.
Pour enchilada sauce evenly over the top, then sprinkle with Monterey Jack cheese and remaining cheddar.
Bake for 15–20 minutes, or until the cheese is bubbly and golden.
Serve with sour cream, avocado slices, or cilantro if desired.