In 10-inch skillet, heat 1-inch water to boiling over high heat. Add onions; if peas are fresh add peas; heat to boiling. Reduce heat; cover and simmer until tender.10-15 minutes.
Meanwhile, prepare white sauce; In heavy 2 quart saucepan, melt butter over low heat. Add flour and cook, stiring, 1 minute. With wire whisk, gradually whisk in hot milk, cook over medium heat, stirring constantly with wooden spoon, until mixture has thickened and boils.Reduce heat and simmer, stirring frequently,3 minutes. Stir in salt and pepper;Remove from heat.Return onions and peas to skillet.Add sauce and cook, stirring, until heated through.