Pat the chicken thighs dry with paper towels. Season both sides with salt, black pepper, paprika, and garlic powder.
Lightly coat the chicken thighs with flour, shaking off any excess.
Heat olive oil and butter in a large skillet over medium heat. Add the chicken thighs and sear for 4 to 5 minutes per side, until golden on the outside. The chicken does not need to be fully cooked yet. Remove it from the skillet and set aside.
Add 2 tablespoons butter and the chopped onion to the same skillet. Cook for 3 to 4 minutes, until the onion is soft and lightly golden.
Add the garlic and cook for about 30 seconds, just until fragrant.
Sprinkle in 1 tablespoon flour and stir for about 1 minute.
Slowly pour in the chicken broth while stirring and scraping up the browned bits from the bottom of the skillet.
Stir in the Dijon mustard, Italian seasoning, and black pepper. Simmer gently for 3 to 4 minutes, until the sauce starts to thicken slightly.
Reduce the heat to low and stir in the Parmesan cheese until melted and smooth. Do not boil the sauce after adding the Parmesan.
Return the chicken thighs to the skillet and spoon the sauce over the top. Simmer gently for 6 to 8 minutes, or until the chicken is fully cooked and tender.
Taste the sauce before adding extra salt. The broth and Parmesan already add salt, so add only up to 1/4 teaspoon if needed.
Finish with lemon juice and fresh parsley. Serve warm with rice, mashed potatoes, pasta, or roasted vegetables.