Add the butter and oil to a large pan. I use my Le Creuset dutch oven. When hot add all of the onions. Turn down heat to medium. Cook the onions until they start to cook down. Place the pan in a 350 degree oven. Cook for 1 1/2 hours stirring a couple of times. The onions should be caramelized by this time.
Divide the ground beef into 4 patties. Cook on the grill or in a pan. When done add the swiss cheese. Top with the caramelized onions.
Put the patties in the buns.
Recipe by Recipes Food and Cooking at https://recipesfoodandcooking.com/2017/05/08/french-onion-sliders-burger-month/