Bring medium-sized saute pan up to medium heat and add oil, onions, garlic and sugar. Saute until onions are soft, about 5 minutes. Add beef bouillon and wine; stir until bouillon dissolves.
Saute until wine is mostly absorbed, about 4-5 minutes. Add mushroom caps and saute until soft, about 5 minutes total, turning to cook each side. Place caps on plate, topping with sauce from the saute pan. Serve hot.