Put the potato and sweet potato slices in two separate pans of water, bring to the boil and cook for 15 minutes.
Meanwhile, heat 1 tablespoon olive oil (if using) in a large frying pan and cook half the onion for 5 minutes, adding in the garlic (if using) for the last minute. Stir through the peas and cook for a further 2-3 minutes. Tip into a bowl with the sliced potatoes, and pour in the beaten eggs, half the chilli and season to taste. Fold carefully together, making sure all the potatoes are covered in the egg mixture.
Heat the butter (if using) in the same pan, pour in the frittata mixture, then layer the sweet potato slices on top. Cook over a low heat for 15-20 minutes
Preheat the grill. Mix together the tomatoes, chilli and remaining onion in a bowl, then season to taste and set to one side.
Brush the sweet potato with the remaining oil, and place the frittata under the grill and cook for a further 5-10 minutes, until fully set. Serve in slices with the tomato salsa and the lettuce.