Preheat oven to 400°F. Pat chicken dry, season with salt and pepper.
Sear thighs skin-side down in an ovenproof skillet with oil until browned, about 6–8 minutes. Flip and remove to a plate.
Sauté onion in the same skillet until soft, add garlic and cook 30 seconds.
Stir in Dijon, cream, broth, thyme, and red pepper flakes; simmer 2 minutes.
Add spinach and Parmesan; stir until wilted and sauce is smooth.
Nestle thighs back into the sauce, skin up, spooning some sauce over.
Bake 20–25 minutes until internal temp reaches 165°F and sauce bubbles.