Prep the chicken. Pat the chicken dry and slice large breasts horizontally into cutlets for even cooking.
Season both sides with Cajun seasoning, a pinch of salt, and pepper.
Sear for flavor. Heat 1 tablespoon oil in a large skillet over medium-high. Add the chicken and sear 3–4 minutes per side until browned and just cooked through. Remove to a plate and tent with foil.
Soften the aromatics. Lower heat to medium.
Add the butter (if using) and a splash of oil if the pan is dry. Sauté onion and bell pepper for 3–4 minutes until softened. Stir in garlic for 30 seconds until fragrant.
Build the base. Stir in tomato paste and cook 1 minute to caramelize.
Pour in chicken broth, scraping up the browned bits. Simmer 2 minutes.
Make it creamy. Reduce heat to low. Whisk in the cream (or Greek yogurt off the heat to prevent curdling), Parmesan, and the zest of half a lemon.
Let it gently thicken for 2–3 minutes. If using yogurt, temper it by whisking in a few spoonfuls of hot sauce from the pan first, then add it back in off heat.
Season to perfection. Taste and adjust with more Cajun seasoning, salt, and pepper. Add a squeeze of lemon juice to brighten.
Finish the dish. Stir in spinach until just wilted.
Return chicken and any juices to the pan, turning to coat. Simmer 1–2 minutes to warm through.
Serve hot. Garnish with chopped parsley or green onions. Spoon over rice, pasta, or roasted vegetables.