Pat the chicken pieces dry and season with salt, pepper, and paprika. Heat olive oil in a large skillet over medium-high heat. Add the chicken in a single layer and cook for 3-4 minutes per side, until golden brown. Remove chicken from the skillet and set aside.

In the same skillet, melt the butter over medium heat. Add the chopped onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for another minute until fragrant. Add the sliced mushrooms, season with a pinch of salt, and cook for 5-7 minutes until they have released their moisture and started to brown.

Sprinkle the flour over the mushrooms and onions, stirring to combine. Cook for 1 minute to toast the flour. Slowly pour in the chicken broth while whisking constantly to avoid lumps. Stir in the Dijon mustard.

Bring the sauce to a simmer and cook for 3-5 minutes, or until it has thickened slightly. Return the cooked chicken and any accumulated juices to the skillet. Reduce the heat to low and let it gently simmer for 5 more minutes.

Remove the skillet from the heat and let it cool for a minute. Stir in the room temperature sour cream until the sauce is smooth and creamy. Do not return to a boil. Season with additional salt and pepper to taste.

Garnish with fresh parsley and serve immediately over hot egg noodles, rice, or mashed potatoes.