In a skillet over medium heat, brown the ground beef until fully cooked. Drain any excess fat.
Add the chopped onion, minced garlic, taco seasoning, and water to the skillet. Stir to combine and cook for about 5 minutes, until the mixture thickens.
Remove from heat and stir in the shredded cheddar and Monterey Jack cheeses until melted.
Preheat your oven to 375°F (190°C).
On a flat surface, place a flour tortilla and spoon some of the beef and cheese mixture in the center.
Fold the sides over the filling and roll tightly to form a chimichanga.
Brush the outside with melted butter or vegetable oil and place on a baking sheet seam side down.
Repeat with the remaining tortillas and filling.
Bake in the preheated oven for 20-25 minutes, or until golden brown and crispy.
Serve hot with sour cream, salsa, and chopped cilantro.