Season the chicken: Pat chicken dry. Season with 1 teaspoon salt, 1/2 teaspoon black pepper, and the Italian seasoning.
Toss to coat.
Sear the chicken: Heat 1 tablespoon olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Add chicken in a single layer. Sear 3–4 minutes per side until golden and just cooked through.
Transfer to a plate and cover loosely.
Sauté the zucchini: Add remaining 1 tablespoon olive oil to the skillet. Add zucchini with a pinch of salt and pepper. Cook 3–4 minutes, stirring occasionally, until lightly browned but still crisp-tender.
Transfer to the plate with the chicken.
Build the flavor base: Lower heat to medium. Add remaining 1 tablespoon butter, onion, and a pinch of salt. Cook 2–3 minutes until translucent.
Stir in garlic and red pepper flakes; cook 30 seconds until fragrant.
Tomatoes and deglaze: Add sun-dried tomatoes and stir. Pour in chicken broth. Scrape up the browned bits from the bottom of the pan.
Simmer 1–2 minutes to reduce slightly.
Make it creamy: Reduce heat to medium-low. Stir in cream and Parmesan. Simmer gently 2–3 minutes, stirring, until the sauce thickens enough to coat a spoon.
Season with salt and pepper to taste.
Add greens and finish: Stir in spinach until wilted, about 1 minute. Return chicken and zucchini (and any juices) to the skillet. Toss to coat.
Add lemon juice and adjust seasoning.
Garnish and serve: Sprinkle with chopped basil or parsley. Serve hot with extra Parmesan and lemon wedges. Great over orzo, rice, mashed potatoes, or cauliflower mash.