Cook onion, 1/8 teaspoon salt, and 1/8 teaspoon pepper in 2 tablespoons butter in a small heavy skillet over moderately low heat, stirring occasionally, until golden brown, about 12 minutes.
Transfer to a bowl, then add cottage cheese, eggs, flour, 1/4 cup butter, remaining 1/8 teaspoon salt, and remaining 1/8 teaspoon pepper and whisk until combined.
Brush a 12-inch nonstick skillet with some of remaining butter and heat over moderate heat until hot but not smoking. Working in batches of 5, scoop 1/8-cup measures of batter into skillet and cook until undersides are golden brown, 1 to 2 minutes. Flip and cook until undersides are golden brown and pancakes are cooked through, 1 to 2 minutes more.
Transfer to a baking sheet and keep warm in oven.
Brush skillet with butter between batches if necessary.
