In a large nonstick covered skillet that has been lightly sprayed with cooking spray, cook onions and garlic over low heat until onions wilt, about 15 minutes.
Stir frequently.
Uncover, stir in sugar, and continue to cook, stirring often, until onions begin to brown.
Meanwhile, in a large saucepan bring stock to a boil.
Add potatoes, reduce heat to a simmer, and cook until potatoes are tender when pierced with a fork, about 15 minutes.