Preheat oven to 375°F; lightly grease 2 (12-cup) muffin pans.
Cook ground chuck and onion in a large skillet over medium heat until beef is browned and crumbly; drain. Stir in refried beans, taco seasoning mix, and 1/4 cup water; bring to a boil, reduce heat, and simmer 10 minutes or until thickened.
Line muffin cups with wonton wrappers; top each with 1 Tbsp meat mixture and 1 Tbsp cheese. Lightly press down and place a second wonton wrapper over cheese. Top with 1 Tbsp meat mixture and 1 Tbsp cheese.
Bake 15 minutes or until edges of wontons are browned and crisp. Top with sour cream and salsa.
Notes
Prepare ground beef ahead of time and store in refrigerator.