Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Cut potatoes into 1/2-inch-thick wedges. Toss on a baking sheet with a large drizzle of olive oil, garlic powder, salt, and pepper. Roast on top rack, tossing halfway through, until browned and tender, 20-25 minutes.

Meanwhile, slice tomato into 1/2-inch-thick rounds. Halve buns. In a small bowl, combine blue cheese dressing and sour cream. Season with salt and pepper.

Season tomato slices all over with salt and pepper. Heat a drizzle of olive oil in a large, preferably nonstick, pan over medium-high heat. Add tomato and cook until lightly browned and slightly softened, 2-3 minutes per side. Turn off heat; transfer to a plate. Wipe out pan.

While tomato cooks, form beef into two patties (four patties for 4 servings), each a bit wider than a burger bun. Season generously with salt and pepper.

Heat a drizzle of olive oil in pan used for tomato over medium-high heat. Add patties and cook to desired doneness, 3-5 minutes per side. In the last 1-2 minutes of cooking, top each patty with Monterey Jack; cover pan until cheese melts.

Toast buns. Spread insides of top buns with half the blue cheese sauce. Fill buns with patties, griddled tomato, and crispy fried onions. Serve with potato wedges and remaining blue cheese sauce on the side.