Although mousselike sabayon is usually eaten sweetened as a dessert, it can also be a savory sauce, as Kristen Murray, pastry chef and owner of Maurice restaurant in Portland, learned from her grandmother. She prefers thin asparagus because they're fun to eat, like skinny fries.
1. Prepare sabayon: In a wide pot, bring 1/2 in. water to a bare simmer; reduce heat to maintain temperature. In a large, wide stainless steel bowl that sits in pot without touching water, whisk grapefruit zest and juice, sugar, and egg yolks to blend. Set over hot water and whisk vigorously with an over-under motion until sabayon doubles in volume and holds a soft shape in bowl, 10 to 12 minutes. Nest bowl in a larger bowl of ice with a little water until cool, 15 minutes.
2. In a medium bowl, beat cream and crème fraîche with a mixer until soft peaks form. Fold into sabayon in thirds, taking care not to deflate. Season to taste with salt. Chill.
3. Meanwhile, prepare asparagus: Preheat oven to 400°. Snap off woody ends of asparagus and trim on a diagonal. Divide asparagus between two rimmed baking sheets. Drizzle evenly with oil and sprinkle with salt and pepper; toss to coat.
4. Roast asparagus until tender-crisp, switching pan positions halfway through, about 8 minutes. Serve hot, with sabayon.
Make ahead: Sabayon, up to 3 hours, chilled.