Using your hands, gently mix onion, grass fed beef, 1¼ tsp. salt, and ½ tsp. pepper in a medium bowl. Gently shape into four 1½”-thick patties (loosely formed patties will be more succulent).
Heat oil in a large skillet, preferably cast iron, over medium-high heat. Season patties with salt and pepper, place in skillet, and immediately reduce heat to medium.
Cook 4–6 minutes per side for medium-rare. Serve on buns with desired toppings.
Notes
Patties can be formed up to 4 hours ahead. Cover and chill until ready to cook.