Rinse your lentils thoroughly and cook in 2 cups of water on low heat, until all water evaporates. Cook under the lid and be careful not to burn.
In the meantime, sauté mushrooms, onion and garlic with black pepper until golden brown.
In a small dish, prepare your flax seed egg and let it sit for 5 minutes.
When ready, transfer the cooked lentils into a bowl and mix with rest of the ingredients.
Mix thoroughly and cover the bowl with plastic wrap or lid and place it into a refrigerator for at least one hour.
When ready, preheat the oven to 400F°, 200C°.
Line your baking sheet with parchment paper and create about ½ cup size patties using.
Arrange them with on your baking sheet and bake for 15 to 20min.
In the meantime, you may grill your onion on each side on with pinch of pepper and pink salt.
Serve with your favorite condiments and veggies.
You can also try grilling your burgers on a grill whether charcoal or electric, although I have not tried that.