In a small bowl, mix 3 Tbsp of flaxseed meal with ¼ cup cold water and let sit while preparing everything else.
Finely chop mushrooms, onions, and mince garlic. If you have a food processor, this is very handy for saving time.
Line a baking sheet with parchment paper and preheat oven to 375° F (if cooking right away).
Heat large skillet on medium heat with 2 tsp avocado oil. Once hot, add onions and mushrooms. Sauté, stirring occasionally, until the mushrooms just begin to stick. Add 3 Tbsp Tamari and stir up any bits stuck to the pan. Stir in garlic and cook until moisture cooks off once again, then turn off heat and set pan aside.
Meanwhile, in a large bowl, add 1 cup of breadcrumbs and everything else under "beans + spices". When mushroom mix has finished cooking, add that to the large bowl along with the flaxseed mixture. Use the food processor to pulse mixture or a potato masher (or even your hands) to mash and combine to desired consistency.
Form patties with your hands. Consistency should be slightly tacky but not sticky. If too sticky, slowly add more breadcrumbs. Careful not to add too much breadcrumbs; if patties are crumbling then that means the mixture is too dry and they'll fall apart easily.
When ready to bake, brush patties lightly with oil. Bake for 20-25 minutes on middle rack, carefully flipping after the first 15 minutes once they are slightly browned, careful not to over-cook!
Once the second side is lightly browned, remove from oven and let rest for 10 minutes before handling. Assemble burgers on lightly toasted buns with toppings and condiments of choice. Enjoy!