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.