Put meat and onion in a food processor, in batches if necessary, and pulse until coarsely ground: finer than chopped, but not much. Put it in a bowl and sprinkle with salt and pepper. Taste, then add more seasoning if necessary. (If desired, cook a teaspoon of meat in a pan before tasting.) Handling meat as little as possible to avoid compressing it, shape it lightly into 4 or more burgers.