Green Bean and Tomato Salad with Yogurt Dressing

1⁄2 cup yogurt
2 teaspoons fresh lemon juice
1⁄2 teaspoon garlic powder
1⁄2 teaspoon onion powder
3 tablespoons olive oil, divided
Kosher salt and freshly ground black pepper 1 pound green beans,  trimmed
1 sweet onion, sliced
1 pint cherry tomatoes, halved
1⁄2 cup thinly sliced fresh mint leaves

Whisk yogurt, lemon juice, garlic powder, onion powder, and 1 Tbsp. oil in a small bowl; season with salt and pepper.
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.
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.
Serve salad drizzled with buttermilk dressing and topped with mint.