Make a flaxseed egg by stirring the flaxseed meal and the water in a small bowl and set aside.
In a food processor, combine 4 ounces of the mushrooms, 1 cup of the cooked lentils, ½ cup of the walnuts (broken), and the seasonings.
Pulse until ingredients are combined. Do not over-pulse.
Now, transfer the mixture into a large bowl.
Then, add the flax egg, bread crumbs, 4 ounces of mushrooms, 1 cup of lentils, and ½ cup of chopped walnuts.
Stir together to combine.
Using a burger press or your hands, form similar-sized burger patties.
Place formed patties on a baking sheet lined with a silicone baking mat or wax paper.
Cover the pan, and place in the freezer for 15 minutes.
If you plan to make the lentil burgers later, place them in the refrigerator and then transfer them to the freezer 15 minutes before cooking them.