In a dutch oven or 4-5 quart saucepan, melt margarine. Add celery, garlic, and onion. Saute until softened, about 5 minutes. Add vegetable stock, peas, and white pepper. Over medium heat, cover and simmer 5 minutes.
Transfer mixture to bowl of food processor. Cool slightly. Puree. Return mixture to pan; add milk and nutmeg. Heat through, about 5 minutes, stirring constantly.
Top each serving with garnish if desired.
VARIATION: stir in 1 carrot, steamed and sliced, additional whole peas, or sauteeed and sliced mushrooms.