Heat sugar in a 12-inch stainless-steel skillet over medium-high heat until completely melted and light brown, about 3 minutes. Immediately add onions and stir with wooden spoon to coat onions in sugar. Add butter, baking soda, 1 teaspoon salt, and a few grinds black pepper. Toss to combine. Cook, shaking pan occasionally until onions release all their liquid and brown coating builds up on bottom of pan, 6 to 8 minutes.
