To save time, the asparagus in this bright salad can be cooked, cooled, dried, and then wrapped up and refrigerated up to a day in advance.
Bring a large pot of salted water to a boil. Prepare a large bowl of ice water, and line a baking sheet with a double thickness of paper towels.
Add asparagus to pot; cook until crisp-tender, 3 to 4 minutes. With a slotted spoon or wire skimmer, transfer to ice bath. Let cool completely, then transfer to prepared baking sheet and pat dry. (Wrap in plastic and refrigerate up to 1 day.)
In a large pot with a lid, heat butter over medium. Add asparagus and peas. Cover and cook, stirring occasionally, until vegetables are heated through, 6 to 8 minutes. Remove from heat; stir in radishes and tarragon. Season with salt and pepper. Serve immediately.