1. Preheat oven to 400°. Heat butter and olive oil in a heavy skillet over medium-high heat. Add onions and salt. Cover; cook 5 minutes. Uncover; add sugar. Cook, stirring frequently, 25 minutes or until onions are golden. Add flour, and reduce heat to medium-low; cook 1–2 minutes. Add sherry, scraping pan to loosen browned bits. Cook 1–2 minutes or until slightly reduced. Transfer to a saucepan. Add stock and pepper. Boil; reduce heat to low, and simmer, covered, 10 minutes. Add soy sauce.
2. Place bread slices on a baking sheet; bake 7 minutes per side or until golden.
3. Heat broiler. Ladle soup into ovenproof bowls. Top with 1–2 bread slices and 2 slices Gruyère. Broil 1–2 minutes or until cheese is bubbly.