Pat the chicken thighs dry. Season both sides with salt, pepper, and smoked paprika, then dust lightly with the flour.

Heat the olive oil in a very large deep skillet over medium-high heat. Place the chicken skin-side down without crowding, working in batches if needed, and cook until deeply golden, about 6 to 8 minutes per batch. Flip and cook 3 minutes more, then transfer to a plate.

Reduce the heat to medium. Add the butter, mushrooms, and onion to the same skillet. Cook, stirring occasionally, until the mushrooms release their moisture and the onions soften, about 7 to 8 minutes.

Add the garlic and tomato paste. Stir for 1 minute until fragrant and slightly darkened.

Pour in the wine or broth and scrape up the browned bits from the bottom of the pan. Simmer for 2 minutes to reduce slightly.

Add the chicken broth, cream, Dijon mustard, and thyme. Stir until smooth, then return the chicken and any juices to the skillet, nestling the pieces into the sauce.

Lower the heat to medium-low, cover partially, and simmer gently until the chicken is tender and reaches 165°F / 74°C in the thickest part without touching bone, about 18 to 22 minutes. Spoon sauce over the chicken a few times as it cooks.

Stir in the spinach in handfuls and cook just until wilted, about 1 to 2 minutes. Taste the sauce and adjust with more salt, pepper, or a squeeze of lemon if needed.

Finish with chopped parsley and serve hot with mashed potatoes, rice, egg noodles, or crusty bread.