Heat the oil in a large frying pan over a medium heat. Add the onion and cook for 5 minutes until soft.
Add the mushrooms and cook for 5 minutes until they have released their liquid and started to brown.
Add the garlic and paprika and cook for 1 minute until fragrant.
Sprinkle in the flour and stir well for 1 minute. Crumble in the stock cube and add the water gradually, stirring constantly.
Stir in the Dijon mustard and drained green lentils. Simmer for 10 minutes until the sauce has thickened.
Remove from the heat and stir through the creme fraiche. Season with salt and black pepper.
Serve over rice or pasta, scattered with fresh dill or parsley.