Preheat oven to 350°F.
In a skillet, melt the butter over medium heat and add the sliced onions. Sauté for about 15 minutes, frequently stirring or turning, until the onions become soft, golden and caramelized.
Add the balsamic, garlic, thyme, paprika and black pepper. Cook for another 2 minutes.
Pour into a baking dish along with the chicken and broth.
Top with baguette pieces and cheese.
Bake for 10-15 minutes, or until bubbly and the cheese is melted. Broil for a few minutes to brown the cheese.
