Cook the bacon in a skillet over medium heat until crisp, about 6–8 minutes. Set aside on paper towels. Leave 1 tablespoon of the bacon fat in the skillet.
Add the sliced onions to the skillet and cook over medium heat until golden and soft, about 8 minutes. Stir occasionally. Remove from the pan.
Form two loose patties with the ground beef. Press them flat and season each side with salt and pepper.
Sear the patties in the skillet over medium-high heat for 3 minutes per side. Lower the heat, place 2 slices of cheese on each patty, cover, and let melt for 1 minute.
Spread Thousand Island on one side of each bread slice.
In a clean skillet, place one slice of bread sauce-side down. Layer with a patty, 2 bacon strips, sautéed onions, and the second slice of bread (sauce-side out).
Cook over medium heat for 3–4 minutes per side until golden and crisp. Press gently with a spatula to help it all come together.
Slice and serve immediately.
