Prep ingredients: Preheat broiler with top rack 6 inches from heat source. Place 1 tablespoon butter in a small bowl and set aside at room temperature to soften until step 6. Finely chop 1 teaspoon garlic. Cut onion into 1-inch pieces. Pat sun-dried tomatoes dry, then coarsely chop.

Season chicken: Pat chicken dry, then pound to ½-inch thickness, if necessary. In a small bowl, combine 1 teaspoon Italian seasoning, 1 tablespoon flour, and a generous pinch each of salt and pepper. Sprinkle seasoning mixture all over chicken, patting to help it adhere.

Cook chicken: Heat 1 tablespoon oil in medium skillet over medium–high. Add chicken and cook until lightly browned and cooked through, 3–4 minutes per side. Transfer to a plate.

Build sauce: Heat 1 tablespoon oil in same skillet over medium. Add onions to skillet; cook, stirring, until softened, about 6 minutes. Add sun-dried tomatoes and ¾ teaspoon of the garlic; cook, stirring, until fragrant, 30 seconds. Stir in chicken broth concentrate and ⅔ cup water. Bring to a simmer; cook, scraping up browned bits, until reduced by ⅓, 3 minutes. Stir in mascarpone.

Add spinach: Add spinach to skillet with sauce. Cook over medium-high heat, stirring occasionally, until spinach is wilted, about 2 minutes. Season to taste with salt and pepper. Return chicken and any resting juices back to the skillet, then keep warm over low heat.

Make garlic bread & serve: Add remaining chopped garlic to bowl with softened butter; mash to combine. Spread butter over cut sides of ciabatta; place on a piece of aluminum foil (or a baking sheet). Broil on top of the oven rack until butter is melted and bread is lightly browned, 1–2 minutes (watch closely as broilers vary). Serve creamy Tuscan chicken with garlic bread alongside.