Heat olive oil in a deep pan over medium heat and add finely chopped onion along with 1/2 teaspoon of salt. Cook for about 5-6 minutes until they turn golden brown and fragrant.
In another pan, melt butter over medium-high heat before adding sliced mushrooms and remaining salt. Sauté for around 5-6 minutes until they are beautifully browned.
Transfer the cooked mushrooms into the pan with onions, then stir in chopped garlic, Dijon mustard, and black pepper for added flavor. Pour in vegetable stock and let it simmer gently for about 5 minutes.
In a small bowl, mix flour with water or additional stock until you achieve a smooth consistency, creating your thickening slurry.
Add cooked lentils to your pan mixture before slowly incorporating your prepared slurry while stirring continuously to ensure even thickness throughout.
Reduce heat to low and gently stir in sour cream until completely blended into your stroganoff sauce; adjust seasoning as needed before serving hot over mashed potatoes, pasta, or rice for an indulgent meal!