Preheat the oven to 375 degrees . In a large skillet, heat the butter and olive oil over medium-high heat. Add the onion and cook until softened, about 5 minutes. Transfer to a large bowl. Mix in the bread, chicken, sun-dried tomatoes and garlic; season with salt.
Spread the chicken-bread mixture in a heavy, ovenproof skillet. Combine the batter and chicken broth in the large bowl and add to the skillet, pressing down to moisten the bread. Bake until golden and set, 30 to 35 minutes.