Peel the onions and slice them evenly into thin wedges, half-moons, or rings, about 1/4-inch thick. Uniform pieces ensure even cooking.
Heat a large nonstick or stainless-steel skillet over medium heat. Add the butter and let it melt until foamy, but not browned.
Scatter the sliced onions in an even layer in the pan—avoid overcrowding to allow for proper browning. Cook in batches if necessary.
Stir the onions to coat them in butter. Sauté for 2–3 minutes, stirring frequently, until the onions are glossy and just starting to soften.
Reduce heat to medium-low. Continue cooking for 5–8 minutes, stirring every minute or 2, until the onions are translucent and develop a light golden color at the edges. If the onions begin to brown too quickly or stick to the pan, lower the heat and add a splash of water.
Season to taste with salt and pepper and serve as desired.