Mince mushrooms finely, or pulse them in a food processor, until they are very finely minced and resemble pebbles.
Cook the mushroom mixture in a dry pan for 5-7 minutes on medium high heat, until moisture is no longer evaporating from them and the mixture has darkened in color. Remove from heat and let cool. Use a paper towel to squeeze any additional moisture out.
Prepare your diced onion and other ingredients. To the bowl with the ground beef, add the mushrooms and the rest of the ingredients. Mix everything together carefully with your hands and form into 6 patties.
Refrigerate the patties for 30 minutes on a paper towel lined plate. This will ensure they don't fall apart on the grill.
Grill the burgers on high heat for 3 minutes on the first side, and an additional 4 min after flipping.
Notes
If you haven't squeezed enough moisture from the mushrooms, you may have issues with the burgers falling apart on the grill or in a pan. Be sure to cook them for long enough and squeeze out any additional excess moisture.
Recipe by Uproot Kitchen at https://uprootkitchen.com/2015/08/10/portobello-beef-burgers/