Place flaxseed, Worcestershire sauce, 1/4 teaspoon kosher salt, cumin, pepper, mushrooms, half of the beans, egg, and garlic in a food processor; process 1 minute or until almost smooth. Place bean mixture in a bowl; stir in remaining half of beans and panko. Heat oil in a large cast-iron or nonstick skillet over medium-high. Fill a 1/2-cup measure with bean mixture; add to pan. Repeat procedure 3 times to form 4 patties. Flatten patties slightly with the back of a spatula. Cook 2 to 3 minutes on each side or until browned. Combine remaining 1/4 teaspoon salt, juice, and avocado in a bowl, mashing with a fork. Stir in yogurt and 1 tablespoon water. Divide patties among bottom halves of buns; top evenly with avocado mixture, cabbage, and top halves of buns.