Peel and dice turnips. Melt the butter (or use olive oil) in a large skillet; add turnips, cover and cook over medium heat for about 10-12 minutes, or until they begin to color--changing from white to a light yellow. Set them aside. Bring the consomme to a simmer in a medium saucepan. Add the turnips and a pinch of sugar. Simmer, skimming if necessary, until the turnips are cooked and soft when piered with a fork, about 10 to 15 minutes. If the peas are frozen, pour boiling hot water over them and let stand for 3 minutes. Drain. If the peas are fresh, blanch in boiling water until bright green, about 1 minute. Drain. Place the peas and croutons in a warmed soup tureen and cover with the hot turnip soup. Serve immediately in heated individual soup bowls.