Ingredients

• 5 tablespoons butter, softened, divided
• 1 tablespoon olive oil
• 1 small onion, finely chopped
• 1 teaspoon salt
• 2 pounds ground beef
• 16 slices American cheese, divided
• 1 package frozen french fries
• 4 thick-cut slices sourdough bread
• sauce:
• 1/2 cup mayonnaise
• 2 tablespoons ketchup
• 1 tablespoon sweet pickle relish
• 1 teaspoon white wine vinegar
• 1 teaspoon salt

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Directions

1. Heat 1 tablespoon butter and oil in a large skillet over medium heat. Add onions; cook 5-6 minutes or until starting to lightly brown. Reduce heat to medium-low. Cook 20-25 minutes, stirring every 5 minutes, until onions are a deep brown color and very soft. Season with salt. Remove from skillet.
2. Shape ground beef into eight 4-oz. patties. Add one beef patty to the skillet. With a heavy metal spatula, flatten to 1/4- to 1/2-in. thickness. Cook until edges start to brown, about 1-1/2 minutes. Turn; top with one slice American cheese. Cook until burger is browned and a thermometer reads at least 160°, about 1 minute. Repeat with remaining beef.
3. Meanwhile, in a small bowl, combine all sauce ingredients. Refrigerate until ready to assemble burgers.
4. Prepare frozen french fries according to package directions; season with salt to taste.
5. To assemble sandwiches, spread one side of each slice of sourdough with 1 tablespoon softened butter. Flip bread over. On two pieces of sourdough, place two slices of American cheese on the non-buttered side of bread. Layer with four cooked burger patties, cooked french fries, caramelized onions and sauce. Top with two more slices of American cheese. Top with a second piece of sourdough, buttered side out.
6. In a skillet over medium heat, toast sandwiches until golden brown on both sides and cheese is melted.