This is an unusual use for pistou, a Provençal variant of pesto made of basil, garlic, olive oil, and salt. It's normally stirred into simple vegetable soups to give them some oomph, but it brings the same lively savoriness to green beans, contrasting nicely with their buttery quality. Don't worry about being knocked over by raw onions on top of all the other strong flavors-soaking them in cold water for 15 minutes removes some of their pungency while preserving their crunch.
Cooks' notes:
Pistou can be made 6 hours ahead and transferred to a small bowl, then chilled, covered. Beans can be cooked 1 day ahead and chilled in a sealed large plastic bag lined with paper towels.
