Prepare a steamer.
Steam Large Portobello Mushroom (8) caps until mushrooms are tender, about 7 minutes.
Place cooked mushrooms in a food processor with minced Garlic (4 clove), Steak Sauce (3 tablespoon), Breadcrumbs (3/4 cup), Egg (1) and Freshly Ground Black Pepper (1/2 teaspoon).
Pulse just until mixture comes together, about 20 pulses or 30 seconds.
Divide mixture into six to eight 6 oz. patties, 4" wide by ⅓ " thick. Place on a plate and refrigerate for 20 minutes or until ready to use.
Thinly slice the Red Onion (1).
Heat Butter (3 tablespoon) in a 12" skillet over medium-high heat. Sauté onions until soft, about 8 minutes.
Add Balsamic Vinegar (2 tablespoon), Dried Thyme (2 teaspoon), Salt and Pepper (to taste). and cook until onions are slightly caramelized, about 7 minutes more. Remove from the pan and reserve.
Wipe out skillet and heat Olive Oil (2 tablespoon) over medium-high. Cook portobello burgers, until toasted on each side and cooked through, about 12 minutes (6 minutes per side).
Top each burger with Blue Cheese (4 ounce) and continue to cook until just melted, about 2 minutes.
Toast the Sesame Hamburger Bun (4) and top with burgers.
Top with onions, and serve with Pickles (to taste). Enjoy!