Heat the olive oil and butter over medium heat in a large skillet. Add the sliced onions and sauté them for 5 minutes until softened.
1 tbsp olive oil, 1 tbsp butter, 1 yellow onion
Add sliced mushrooms, minced garlic, and ground black pepper. Cook for 5 minutes until tender and fragrant.
1 garlic clove, 9 oz brown mushrooms, 1/4 tsp black pepper
Sprinkle flour over the mushroom mixture and stir it in until everything is evenly coated.
2 tbsp all-purpose flour
Gradually stir in the stock until smooth. Simmer until the sauce has thickened.
1 1/2 cups vegetable stock
Stir in Dijon mustard and sour cream. Season to taste with salt and black pepper and serve immediately.
2 tsp Dijon mustard, 1/2 cup sour cream, salt & pepper
Notes
Adjust consistency: Add more stock if the sauce is getting too thick and more flour to thicken the sauce.Storage: Store leftovers in an airtight container in the refrigerator for up to 4-5 days. I don't recommend freezing this sauce as the texture will change once thawed.