If using uncooked lentils, put in a pan and cover with plenty of cold water. Bring to the boil and simmer, covered, for 15-20 minutes, until tender but still retaining their shape.
Meanwhile, process the mint leaves, pine nuts, garlic, olive oil and lemon zest into a rough pesto. A little texture is good - it shouldn't be totally smooth. Then add the lemon juice and season the pesto generously.
Drain the lentils well, tip into a serving dish and stir the pesto through while still warm. Serve immediately.
If using ready-cooked lentils, simply heat following the pack instructions and stir the pesto through as above.