- 1 tablespoon butter, or as needed
- 1 1/4 cups chopped sweet onion (such as Vidalia)
- 2/3 cup mayonnaise
- 2 cups shredded Swiss cheese
Preheat oven to 350 degrees F (175 degrees C).
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Melt butter in a skillet over medium heat. Cook and stir sweet onion in hot butter until browned, 5 to 7 minutes; transfer to a bowl. Add mayonnaise to the onion and stir; pour into a 9×13-inch baking dish. Sprinkle Swiss cheese over the mixture.
Bake in preheated oven until the top is bubbling and lightly golden, about 35 minutes.