Prep & Season Chicken: Slice the chicken breasts horizontally to create 4 equal-sized thin cutlets. Season both sides evenly with garlic powder, salt, and black pepper. Lightly dredge each cutlet in the flour, shaking off any excess.
Sear to Golden: Heat the olive oil and butter in a large skillet over medium-high heat. Once shimmering, add the chicken cutlets in a single layer and sear for 4-5 minutes per side until golden brown and cooked through (internal temperature reaches 165°F (74°C). Transfer chicken to a plate and set aside.
Deglaze the Skillet: Reduce heat to medium. Pour in the chicken broth (or wine) and Dijon mustard. Use a wooden spoon to scrape up all the delicious browned bits (fond) stuck to the bottom of the pan. Let it simmer for 1-2 minutes.
Build the Sauce: Lower the heat slightly and stir in the heavy cream, baby spinach, and sliced sun-dried tomatoes. Return the cooked chicken cutlets and any rested juices back into the skillet.
Simmer & Serve: Let the mixture simmer gently for 4-5 minutes until the spinach wilts and the sauce thickens to a glossy consistency. Stir in the fresh basil, adjust seasoning with extra salt and pepper if needed, and serve hot!