In a small skillet, saute sweet onion in 1 tablespoon butter until tender. Add vinegar; cook and stir until liquid is evaporated. Cool.
In a small bowl, combine the flour, sugar, baking powder and seasonings. Cut in remaining 3 tablespoons butter until mixture resembles coarse crumbs. Add the onion mixture, cheese and green onions. Stir in buttermilk just until moistened.
Turn onto a lightly floured surface; knead 3-4 times. Pat or roll out to 1/2-in. thickness; cut with a floured 2-1/2-in. biscuit cutter. Place in a greased cast-iron or other ovenproof skillet. Bake at 400° until golden brown, 12-15 minutes. Serve warm.