Heat the olive oil in a large skillet over medium-high heat. Add the onions and red peppers, and cook until softened and the onions are starting to brown, about 5 minutes. Add the vegetarian crumbles and another splash of olive oil, and season with a little garlic powder and seasoning salt. Cook for another 5 to 6 minutes, until the crumbles are warmed through. Add 1 cup water; cook 5 minutes more.
In a bowl, stir together the 1 tablespoon garlic powder, 1/2 tablespoon seasoning salt and the mustard, soy sauce, brown sugar and ketchup. Stir this into the veggie crumble mixture. Serve on the warmed hamburger buns.