Slice the chicken breasts horizontally to create thinner cutlets. Season both sides with salt, black pepper, and garlic powder.
Heat the olive oil in a large skillet over medium-high heat. Sear the chicken for 4–5 minutes per side, until golden and cooked through. Remove from the skillet and set aside.
Reduce the heat to medium and add the reserved sun-dried tomato oil to the same skillet. Add the sun-dried tomatoes, garlic, and Dijon mustard. Cook for about 1 minute, until fragrant.
Pour in the chicken broth and heavy cream, then stir in the black pepper and thyme. Bring to a gentle simmer and reduce the heat to low-medium.
Stir in the parmesan cheese and let the sauce simmer for 2–3 minutes, stirring continuously, until smooth and slightly thickened.
Add the baby spinach and stir just until wilted. Return the chicken to the skillet, spoon the sauce over the top, cover, and simmer for about 2 minutes.
Remove from heat and serve warm.
Notes
Use oil-packed sun-dried tomatoes for the best flavor. Stir the sauce gently after adding parmesan to keep it smooth and creamy. If the sauce thickens too much, add a splash of warm chicken broth to loosen it.