melt 2 tablespoons butter in a large cast-iron skillet over medium heat.

add the onions and cook until translucent, 6 to 8 minutes.

add 1/3 cup water, cover and continue cooking until the onions are golden brown, about 15 more minutes.

season with salt and pepper.

transfer the onions to a bowl and toast buns in the pan

wipe out the skillet.

divide the meat into 4 loose balls, then gently flatten into thin patties.

heat the skillet over high heat.

add the patties to the hot skillet, smash them flat with a spatula and season with salt and pepper.

cook, flipping once, until well done, about 3 minutes per side.

put the patties on the bottom buns and top each with 1/4 cup sautéed onions.

smear 1 tablespoon butter on the cut side of each top bun, then put on top of the burgers.

slice in half and serve immediately.