Preheat oven to 375° if planning on cooking your burgers right away.
Put all of the crackers and seasoning, and approximately one-half of the lentils, mushrooms, and walnuts, into a food processor fitted with the S blade. Process until smooth and the mixture forms a ball.
Turn the food processor off and add in the rest of the lentils, mushrooms, and walnuts, and the flax egg. Pulse until the mixture is mostly smooth but still has some visible bits of walnuts and mushrooms.
Form your burger ‘dough’ into patties on a cookie sheet lined with parchment paper, you should get about 8 patties.
Place the burgers, still on the cookie sheet, into your preheated oven and bake for a total of about ½ hour, flipping the burgers over after 15 minutes.
Serve as you would any other burger.
