How to Make It
Set up an ice bath. In a large saucepan of salted boiling water, cook the asparagus and peas until crisp-tender, about 2 minutes. Drain and transfer the vegetables to the ice bath to cool completely; drain well.
Fill the saucepan with water and return to a boil; season generously with salt. Add the pasta and cook until al dente. Drain well and transfer to a large bowl.
In a medium bowl, whisk the buttermilk with the mayonnaise, vinegar and garlic. Season with salt and pepper.
Toss half of the dressing with the warm pasta; let cool for 30 minutes. Stir in the asparagus, peas and remaining dressing. Season with salt and pepper, then fold in the arugula and serve.