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 garlic clove, 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