01 - Pat chicken breasts dry. Season both sides generously with salt, black pepper, and dried Italian herbs.
02 - Heat olive oil in a large, deep skillet or Dutch oven over medium-high heat. Sear the seasoned chicken breasts for 3 to 4 minutes per side until a golden-brown crust forms. Remove the chicken to a separate plate; it will finish cooking later.
03 - In the same skillet, add the cubed butternut squash and sauté for 4 minutes. Add the sliced red onion and mushrooms; cook for an additional 3 minutes, stirring occasionally, until softened.
04 - Stir in the minced garlic and cook for 1 minute until fragrant, ensuring it does not burn.
05 - Add the sliced sun-dried tomatoes, halved cherry tomatoes, and low-sodium chicken broth to the skillet. Bring the mixture to a simmer and cook for 5 minutes, allowing the squash to begin softening.
06 - Reduce the heat to medium-low. Stir in the heavy cream, grated Parmesan cheese, and Dijon mustard until well combined. Return the seared chicken breasts to the skillet, nestling them into the sauce and vegetables.
07 - Cover the skillet and simmer gently for 12 to 15 minutes, or until the chicken is cooked through (internal temperature reaching 165°F or 74°C) and the butternut squash is tender.
08 - Stir in the baby spinach and red pepper flakes (if using). Continue to simmer just until the spinach has wilted. Taste and adjust seasoning as necessary.
09 - Serve the One-Pot Creamy Tuscan Chicken hot, spooning the rich sauce and vegetables over the tender chicken.