Heat olive oil in a large skillet over medium heat. Add chopped onions and sauté until translucent (about 3 minutes). Stir in minced garlic and cook until fragrant.

Add sliced mushrooms to the skillet. Stir occasionally until they release moisture and turn golden brown (about 7 minutes).

Sprinkle flour over the mushroom mixture and stir well. Gradually pour in vegetable broth while stirring to avoid lumps. Let it simmer for about 5 minutes until thickened.

Reduce heat to low, stir in sour cream and Dijon mustard until fully incorporated. Season with salt and pepper to taste.

Serve hot over noodles or rice, garnished with fresh parsley if desired.