Heat the olive oil in a frying pan and fry the bacon until golden and crisp. Add the lentils, peas and stock. Bring to a rapid simmer to heat through and season with salt, pepper and lemon juice. Stir in the butter to thicken and finally stir in the mint.
Serve hot as an accompaniment to a piece of fried or grilled fish such as cod, haddock or salmon, or eat simply as it is.