Cut 1 large sweet onion lengthwise into thin slices. Thinly slice 1 red bell pepper. Melt 2 Tbsp. salted butter in a large nonstick skillet over medium-high; add sliced onion, and sprinkle with 1 tsp. granulated sugar. Cook, stirring often, 15 minutes. Add bell pepper slices, and cook, stirring often, until onions are browned and tender and pepper slices are tender, about 10 minutes. Remove from heat, and sprinkle with 1/4 tsp. kosher salt and 1/8 tsp. black pepper.