Spray 3 1/2 to 4-quart slow cooker with nonstick cooking spray. Layer half each of potatoes, onions and garlic in sprayed slow cooker. In medium bowl, combine Alfredo sauce, half-and-half, salt and pepper; mix well. Spoon half of mixture over top. Layer with remaining potatoes, onions, garlic and sauce mixture. Do not stir.
Cover; cook on high setting for 3 to 4 hours.
About 30 minutes before serving, sprinkle peas over potato mixture. Cover; cook on high setting an additional 20 to 30 minutes. Stir gently to mix peas with potatoes before serving.