Heat the oven to 350 F. In a small saucepan, saute the onions and garlic until tender, about 5 to 7 minutes. Remove from heat and place in a medium bowl. Add the cheese, Italian seasoning, fennel, salt and pepper; mix well. Add the chicken and gently mix with hands until just combined. Form mixture into 1-inch meatballs —about 3/4 ounces each. Place on a greased baking sheet and bake until the internal temperature of the meatballs reach 165 F, about 15 minutes.