1. Preheat oven to 400 degrees. Place mini bagel halves, crust side down, in a single layer along the bottom of a 5 quart baking dish.
2. In a large skillet over medium-high heat, sauté 1 tablespoon of the butter and onions until onions begin to caramelize, about 7 minutes.
3. Add chicken, hot sauce and remaining butter to the skillet and stir all to combine.
4. Tope each bagel half with an equal amount of the chicken and onion mixture from the skillet. Sprinkle crumbled bleu cheese over all, and then top with mozzarella cheese.
5. Bake 12-15 minutes, or until bagel bottoms are browned and crispy, and cheese is melted. Serve hot.