Melt plant-based butter on a skillet until sizzling. Add onions and cook over medium heat, stirring occasionally (35-40 min). Once tender and deep golden brown, stir in thyme, vinegar, and season to taste with salt and pepper. Continue cooking, stirring occasionally, 5 minutes or until onions are fully caramelized.
Combine Impossible meat, mustard, and salt in a bowl. Mix lightly then shape into 6 1/4-1/2 inch thick patties. Season each side of the patties with pepper.
Place patties on plancha for 90 seconds each side over medium heat. While burgers cook toast hamburger buns. Once the burgers are browned on either side, place Vertage Blue Cheese on top of the Impossible patties and remove off heat.
Assemble burgers layering lettuce, pickles, tomato, patties, and ¼ cup of caramelized onions in between each set of buns.