Heat oven to 350°F. Spray 24 mini muffin cups with cooking spray. Separate dough into 4 rectangles; press perforations to seal. Cut each rectangle into 6 squares. Press each square in bottom and up side of mini muffin cup. Bake 6 minutes. Remove from oven. Using handle of wooden spoon, immediately make 1 1/2 –inch indentation in center of each cup.
Meanwhile, in 10-inch skillet, heat oil over medium heat. Add onion; cook 2 to 3 minutes or until tender. Stir in chipotle chili and the adobo sauce, meatballs and 2 tablespoons water. Cover; reduce heat to low. Cook 3 to 4 minutes, stirring occasionally or until thoroughly heated. Remove meatballs from skillet. Spoon 1/2 teaspoon of the sauce into each cup. Top each with one meatball half, cut side up.
Cut each piece string cheese lengthwise in half; cut each half crosswise into thirds. Top each meatball with 1 piece string cheese. Bake 6 to 10 minutes longer or until edges are golden brown. Cool 1 minute; remove from cups.
Top each meatball cup with 1 avocado slice; secure with wooden toothpick. Serve warm.