Season the cod with sea salt and black pepper, place on an oiled baking tray and lay two slices of pancetta over the top of each fillet.
Halve the lemons and place the four lemon halves, cut side down, on the tray next to the fish. Roast in the preheated oven for 10 to 15 minutes, or until the fish flakes easily.
Peel and quarter the potatoes and cook until soft in boiling salted water. Meanwhile, cook the peas to packet instructions, then drain and blitz the peas in a food processor (or mash up with a fork if you don't have one).
Deseed and finely dice the red chilli, if using.
Drain the potatoes and mash with butter, hot milk, salt and pepper, whizzed-up peas and the red chilli if using.
Put the rocket in a mixing bowl, pick in the mint leaves and toss together and dress with extra virgin olive oil.
Serve each piece of cod on a dollop of pea and potato mash, with a little salad on the side.