Wash and dry all produce. Adjust rack to middle position and preheat oven to 450 degrees. Cut potatoes into 1/2-inch-thick wedges. Halve and peel onion. Thinly slice one half; grate remaining half until you have 1 TBSP (save the rest for another use). Zest 1/4 tsp zest from lime, then cut into quarters. Thinly slice tomato.

Toss potatoes on a baking sheet with a large drizzle of olive oil, half the Fry Seasoning (we’ll use the rest later), and a few big pinches of salt and pepper. Roast in oven until browned and tender, about 25 minutes, tossing halfway through.

Meanwhile, in a small bowl, toss together sliced onion, juice from 3 lime wedges, 1/4 tsp sugar, and a pinch of salt and pepper. Set aside to pickle, tossing occasionally. In another small bowl, stir together sour cream, lime zest, 1/2 tsp sugar, juice from remaining lime quarter, and 1 tsp water. Add sriracha to taste. Season with salt.

In a large bowl, mix beef, grated onion, and remaining Fry Seasoning. Using your hands, gently shape mixture into two evenly-sized patties, each slightly wider than the buns. Season with plenty of salt and pepper.

Heat a large drizzle of oil in a large pan over medium-high heat (we used nonstick). Add patties and cook 4 minutes on one side, then flip and top with cheddar. Cook until browned and cooked to desired doneness, 3-5 minutes more.

Drain liquid from pickled onion. Halve buns. (TIP: Toast buns if desired.) Spread as much crema as you like onto bottom halves. Fill buns with patties, tomato, and onion. Divide burgers and potatoes between plates. Serve with any remaining crema for dipping.