Preheat air-fryer to 400°F. Heat oil in a medium nonstick pan over medium-high heat. Add shallot and cook until softened, 1 to 2 minutes. Add garlic and cook just until fragrant, 1 minute. Remove from heat. In a large bowl, combine panko and milk. Let stand 5 minutes. Add cooked shallot and garlic to panko mixture, along with beef, turkey sausage egg, parsley, rosemary, thyme, mustard, and salt. Stir to gently combine. Gently shape mixture into 1 1/2-inch balls. Place shaped balls in a single-layer in air-fryer basket. Cook half the meatballs at 400°F until lightly browned and cooked-through, 10 to 11 minutes. Remove and keep warm. Repeat with remaining meatballs. Serve warm meatballs with toothpicks as an appetizer or serve over pasta, rice, or spiralized zoodles for a main dish.