Shepherd's Pie With Buttermilk-Chive Mashed Potato Crust

Ingredients

• 1 lb yukon gold potatoes, peeled and cut into 2-inch pieces
• 1 clove garlic, whole, plus 1 tsp garlic, minced, divided
• 2/3 cup buttermilk
• 1 tbsp chives, chopped
• sea salt and ground black pepper, to taste
• 4 tsp olive oil, divided
• 1 lb lean ground turkey breast
• 1 yellow onion, finely chopped (1 cup)
• 3 medium carrots, halved lengthwise and cut into half moons (about 1 cup)
• 1 celery stalk, diced (about 1/3 cup)
• 1 tsp fresh rosemary leaves, finely chopped
• 1 cup low-sodium chicken broth
• 1 tbsp tomato paste
• 1/2 cup frozen peas