Step 1

Whisk buttermilk, lemon juice, garlic powder, onion powder, and 1 Tbsp. oil in a small bowl; season with salt and pepper.

Step 2

Cook beans in a large pot of boiling salted water until just tender, about 3 minutes. Using a slotted spoon, transfer to a colander set in a bowl of ice water; drain. Place beans in a large bowl.

Step 3

Return water in pot to a boil and cook leeks until just tender, about 3 minutes; drain. Transfer to colander in ice water; drain and add to bowl with beans. Add cherry tomatoes and remaining 2 Tbsp. oil and toss to combine; season with salt and pepper.

Step 4

Serve salad over heirloom tomato slices drizzled with buttermilk dressing and topped with mint and chives.