In a deep frying pan, heat 2 tablespoons of olive oil over medium heat. Add one chopped onion and 1 teaspoon of salt. Sauté for 5-6 minutes until golden brown.

In a separate skillet, melt 1 tablespoon of butter over medium heat. Add 1.5 cups of sliced mushrooms and cook for 5-6 minutes until browned.

Transfer sautéed mushrooms to the pan with onions. Stir in 2 minced garlic cloves, 2 teaspoons of Dijon mustard, and ¼ teaspoon of black pepper. Pour in 1.5 cups of vegetable stock and simmer for 5 minutes.

Combine 2-3 tablespoons of flour with a splash of water or stock in a small bowl to create a slurry. Set aside.

Add 1.5 cups of cooked lentils to the mixture and stir well. Gradually incorporate the flour slurry, stirring continuously until the sauce thickens (about 3-5 minutes).

Reduce heat to low and stir in ½ cup of sour cream. Allow to heat through for a couple of minutes, adjusting seasoning as necessary.

Garnish with freshly chopped parsley and serve hot over mashed potatoes, pasta, or rice.