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preheat oven to 425 degrees.
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halve, peel, and finely dice onion.
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cut potatoes into ½-inch wedges.
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halve, peel, and finely mince shallot.
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toss potatoes on a baking sheet with a drizzle of oil, ¾ tsp garlic powder (you’ll use the rest in the next step), salt, and pepper.
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roast on top rack of oven until browned and crisp, 25-30 minutes.
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meanwhile, in a small bowl, combine minced shallot, vinegar, 1 tsp sugar, and a pinch of salt.
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in another small bowl, combine ketchup, mustard, sour cream, 1 tsp sugar, and remaining ¼ tsp garlic powder.
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heat a drizzle of oil in a large pan over medium-high heat.
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add onion and cook, stirring occasionally, until browned and softened, 5-7 minutes.
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season with salt and pepper.
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turn off heat; transfer to a plate.
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while onion cooks, form beef into 2 patties, each slightly wider than a bun.
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season generously with salt and pepper.
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heat a drizzle of oil in same pan used to cook onion over medium-high heat.
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add patties and cook almost to desired doneness, 3-5 minutes per side.
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top with cheddar and cover until melted and patties are cooked to your liking, 1-2 minutes.
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while cheese melts, toast buns in oven or toaster oven until golden brown.
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drain pickled shallot, discarding liquid.
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stir shallot into bowl with sauce.
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fill buns with patties, onions, lettuce and sauce.
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serve any extra sauce on the side for dipping potatoes into.