Combine the ground flax seeds with the water and put aside to soak. In a large bowl, mash the chickpeas with a fork, leaving some chickpeas intact for texture.
Put 1/2 cup of rolled oats into a blender or food processor and ground into a fine powder of flour by pulsing several times.
In a small pan, heat some oil and sauté the onions and the garlic for about 3 minutes. Together with the remaining ingredients, add them to the chickpeas and mix well. Form four large burger patties (or 6 smaller ones).
Line a baking tray with parchment paper and put the burger patties on top. Bake for 30 minutes at 350 °F. Flip them after the first 15 minutes.
Cut the whole wheat buns in halves and cover both halves with the sunflower tomato spread. Alternatively you can of course also use ketchup, mustard, or any other burger sauce. Serve the mushrooms burgers with tomatoes, lettuce, and red onions.