Pat chicken pieces dry with paper towels. Season generously with salt, black pepper, and smoked paprika. Heat olive oil in a large skillet over medium-high heat until shimmering. Add chicken in a single layer and sear for 5-6 minutes, turning occasionally, until golden-brown and cooked to 165°F. Transfer to a plate.

Reduce heat to medium and add butter to the same skillet. Once melted, add diced onion and sliced mushrooms. Sauté for 6-8 minutes, stirring occasionally, until mushrooms release their liquid, it evaporates, and both vegetables turn golden brown. Add minced garlic and cook for 1 minute until fragrant.

Sprinkle flour evenly over the mushroom mixture. Stir constantly for 1 minute to cook the flour and coat all the vegetables.

Slowly pour in chicken broth while scraping the bottom of the pan to release browned bits. Stir until smooth with no flour lumps. Add Dijon mustard and stir to incorporate.

Reduce heat to low. Temper the sour cream by whisking a few spoonfuls of hot broth mixture into the sour cream in a separate bowl, then whisk the warmed cream back into the skillet. Simmer gently for 2-3 minutes, stirring occasionally, until sauce thickens slightly. Do not let it boil vigorously.

Return seared chicken and any accumulated juices to the skillet. Toss to coat thoroughly and heat through for 2 minutes. If sauce is too thick, thin with a splash of broth. Taste and adjust seasoning with salt and pepper.

Remove from heat and garnish with fresh chopped parsley. Serve immediately over egg noodles, rice, or mashed potatoes.