Heat 2 tablespoons of olive oil in a deep frying pan over medium heat. Add the finely chopped onion and a pinch of salt. Sauté for 5-6 minutes until golden brown and translucent.

In a separate skillet, melt 2 tablespoons of butter over medium-high heat. Add sliced mushrooms and sauté for 5-6 minutes until browned and tender, then transfer to the onion pan.

Add sautéed mushrooms to the onion mixture, followed by 2 minced garlic cloves, 1 tablespoon of Dijon mustard, and black pepper. Pour in 1 cup of vegetable stock and simmer for 5 minutes.

In a small bowl, mix 2 tablespoons of flour with 1/4 cup of water until smooth. Add the slurry to the mushroom mixture while stirring continuously.

Stir in 1 cup of cooked lentils and cook for 3-5 minutes until the sauce thickens.

Lower the heat and gently stir in 1/2 cup of sour cream, mixing well until fully incorporated.

Serve over mashed potatoes, pasta, or rice, garnished with freshly chopped parsley.