Heat olive oil in a skillet over medium heat. Cook bratwurst in hot oil until browned, 2 to 3 minutes per side. Remove bratwurst to a plate, reserving drippings in the skillet.

Stir apple and onion slices into the reserved drippings; cook and stir until onion softens, about 6 minutes. Return bratwurst to the skillet, nestling them into the onion mixture. Pour beer over sausages; season with salt and pepper.

Bring beer to a simmer, reduce heat to medium-low, and cook until bratwurst is no longer pink in the center, 25 to 30 minutes. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).