Place 1 cup peas in microwave-safe bowl;set aside.
Heat oil in heavy large saucepanover medium-high heat.
Add shallots andsauté until golden and almost tender, about7 minutes.
Add remaining peas, 4 cupsbroth, and 2 tablespoons tarragon; bring toboil. Reduce heat to medium and boil untilflavors blend and peas are tender, about 7minutes. Cool slightly. Working in batches,puree soup in blender until completelysmooth. Return soup to same saucepan.Bring to simmer and thin with more brothby 1/4 cupfuls, if desired. Stir in remaining1 tablespoon tarragon. Season with pepper.
Cook reserved 1 cup peas in microwaveuntil warm, about 1 minute.
Ladle soup into 6 bowls.
Drizzle lightlywith yogurt.
Sprinkle whole peas over andgarnish with pea sprouts.