In a small bowl, whisk together the buttermilk, mayonnaise, lemon juice, dill, parsley, garlic powder, salt, and pepper until smooth. Set aside to let the flavors meld.
In a large bowl, combine the mixed salad greens, fresh peas, cherry tomatoes, red onion, and feta cheese.
Drizzle the olive oil over the salad mixture and toss gently to combine.
Pour the buttermilk herb dressing over the salad and toss again until everything is well coated.
Serve immediately for the best texture, or chill in the refrigerator for up to 30 minutes before serving to enhance the flavors.