Shepherds Pie with Tater Tot Topping

Name: Shepherd's Pie with Tater Tot Topping
Ingredients: 6 tablespoons unsalted butter, plus for buttering casserole dish; 2 tablespoons olive oil; 3 russet potatoes; Kosher salt and black pepper; 4 spring onions or 1 medium yellow onion, chopped; 4 cloves garlic, smashed and roughly chopped; 3 pounds 80/20 ground beef; 1/4 cup chili powder; One 28-ounce can diced tomatoes, drained; 2 cups beef broth; 2 tablespoons all-purpose flour; 1 1/2 cups fresh or thawed frozen peas; 12 ounces extra-sharp Cheddar, grated; 1/4 cup finely grated Parmesan