Instant Pot Shepherds Pie

Name: Instant Pot Shepherd's Pie
Ingredients: 1 tablespoon avocado oil or other neutral oil with high smoke point; 1 pound ground lamb or beef; 1 yellow onion , diced; 1 celery stalk , diced; 1 large carrot , diced; 1 clove garlic , minced; 1 1/4 teaspoons kosher salt; 1 teaspoon dried thyme; 1/2 teaspoon freshly ground black pepper; 1 tablespoon Worcestershire sauce; 1 tablespoon tomato paste; 1 cup vegetable broth; 1 cup frozen peas; 4 medium russet potatoes , peeled (about 2 1/2 pounds); 1/2 cup whole milk