Preheat the oven to 200°C/gas mark 6. Pat the defrosted peppers dry with kitchen paper.
Simmer the potatoes in a pan of boiling water for 10 minutes, then drain and leave to cool slightly before slicing thinly.
Meanwhile, heat a 25cm non-stick ovenproof frying pan, then add the bacon lardons and dry-fry for 5 minutes until browned. Remove using a slotted spoon and set aside. Add the onion and garlic to the pan and fry in the bacon fat for 5 minutes until softened. Add the peppers and heat through. Stir through the lardons, then transfer to a bowl.
Lightly beat the eggs and add to the bowl, then carefully fold in the potatoes and peas and mix together. Pour the frittata mixture into the frying pan and cook over a low-medium heat for 8-10 minutes. Transfer to the oven and bake for 20-25 minutes. Slice and serve with the torn lettuce.