Beef and Spinach Shepherds Pie

Name: Beef and Spinach Shepherd's Pie
Ingredients: 2 lb. Russet potatoes (about 4), peeled and cubed; kosher salt; 1 lb. lean ground beef; 1 medium yellow or white onion, diced; 2 carrots, peeled and diced; 2 ribs celery, diced; 2 cloves garlic, minced; 2 tsp. fresh thyme leaves; 2 tsp. chopped fresh rosemary; Freshly ground black pepper; 3 tbsp. tomato paste; 1 c. chicken broth; 4 c. baby spinach; 1 c. milk; 3 tbsp. extra-virgin olive oil, plus more for drizzling; Green salad, for serving