Preheat the olive oil and butter in a large pan over medium-high heat. Add the chicken, season with salt and pepper, and cook until lightly browned. Transfer to a plate.
1 tablespoon olive oil, 1 tablespoons butter, 1 pound chicken breast, salt and black pepper
Reduce heat to medium. Add the remaining butter, then add the mushrooms. Let them sit for a few minutes to brown before stirring. Continue cooking until the mushrooms are nicely golden.
1 tablespoons butter, 10 ounces brown mushrooms
Stir in the diced onion and cook for 2–3 minutes until softened. Add the garlic and cook for 30 seconds until fragrant.
½ yellow onion, 2 garlic cloves
Add the tomato paste and sauté for 1 minute to deepen the flavor.
1 tablespoon tomato paste
Sprinkle the flour over everything and stir until evenly coated. Slowly pour in the chicken broth while stirring. Add thyme, then simmer for 10 minutes.
2 tablespoons all-purpose flour, 2 cups chicken broth, ½ teaspoon dried thyme
Reduce heat to low. Stir in the sour cream and Dijon until smooth.
½ cup sour cream, 2 teaspoons Dijon mustard
Add the chicken (and any juices) back into the pan. Let it simmer gently for 10 minutes, until the chicken is cooked through and the sauce looks glossy. If it gets too thick, add a splash more broth.
Spoon over egg noodles (or toss everything together). Finish with chopped parsley or dill and enjoy!
parsley or dill (for garnish), egg noodles (for serving)