In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Add the minced garlic and cook for an additional minute until fragrant.
Season the chicken thighs with salt, black pepper, and cayenne pepper (if using). Add the chicken to the skillet and brown on both sides, about 5 minutes per side.
Sprinkle the sweet paprika over the chicken and stir to coat evenly. Pour in the chicken broth, bring to a simmer, then reduce the heat to low. Cover and let it cook for about 20 minutes, or until the chicken is cooked through and tender.
In a small bowl, whisk together the sour cream and flour until smooth. Gradually stir this mixture into the skillet, cooking for an additional 5 minutes until the sauce thickens. Adjust seasoning if necessary.
Serve the chicken paprikash over cooked egg noodles or rice, and garnish with fresh parsley.