Melt the butter in a skillet; add the onion. Cook, covered,
over medium heat until soft and transparent. Uncover, stir in
the sugar and reduce the heat; cook gently until brown, about
1 hour. Stir in the Madeira and cook 2 minutes longer.
Toss the sauce with drained, cooked pasta. Serve immediately with grated Parmesan cheese.
Recipe adapted from "The New James Beard"