Cook the peas: If using fresh peas, simmer them in lightly salted water or chicken stock until tender (about 20 minutes), then drain and let them cool. Don't overcook the peas; the goal is tender but not mushy.
Prep the veggies: Dice the tomatoes, cucumber, and red onion. Shuck the corn and slice off the kernels.
Toss the salad: In a large bowl, combine peas, tomatoes, corn, cucumber, red onion, and herbs.
Make the dressing: In a small jar or bowl, whisk together buttermilk, mayo, vinegar or lemon juice, chives, garlic, salt, and pepper.
Combine and chill: Pour the dressing over the salad, toss gently to coat, and chill for at least 30 minutes before serving.
