Green Garlic and Pea Soup with Whipped Cream

Cut dark-green tops from green garlic and slice crosswise 1/2" thick; slice bulbs and pale-green stalks crosswise 1/4" thick. Heat 2 tablespoons oil in a large saucepan over medium heat. Add bulbs and stalks, season with salt and pepper, and cook, stirring often, until beginning to soften, 8 –10 minutes. Add tops and cook, stirring often, until soft, about 3 minutes. Add 4 cups water to saucepan and bring to a boil. Add peas, reduce heat, and simmer gently until peas are bright green and tender, about 5 minutes. Let cool slightly, then purée soup in a blender with remaining 2 tablespoons oil, adding water to thin if needed, until smooth. Strain soup through a fine-mesh sieve into a clean saucepan, pressing on solids; season with salt and pepper. Reheat briefly (to preserve color) before serving. Beat cream with a whisk in a medium bowl to soft peaks; season with salt and pepper. Serve soup with a swirl of whipped cream. DO AHEAD: Soup can be made 1 day ahead. Cover and chill.