Rinse the lentils thoroughly & place in a large pot with sufficient cold water to cover by a few inches. Set over a medium-high heat and bring up to a boil. Meanwhile, cut the onion into large wedges (don’t to peel it), cut the garlic bulb in half crosswise (don’t remove the papery skins), and thinly slice the lemon.
Once the water comes to a boil, reduce to a low heat and skim off any impurities from the surface. Add the onion, garlic, lemon slices, herbs, a big pinch of chilli flakes and a very good glug of olive oil (about 3 tbsp) & season with salt and pepper. Be bold with the seasoning: once the salt has dissolved and settled taste the cooking water. It should be notably seasoned.
Cook at a bare simmer for 15-30 minutes, depending on the variety and age. As a general guide, green lentils take around 15-20 minutes, brown lentils take around 20 minutes and darker varieties such as Puy or Beluga take around 25-30 minutes. Check in regularly for tenderness. Remove from the heat when they have transformed from hard pellets to buttery, well seasoned beads with texture.
Drain the lentils & save the cooking liquid to use as stock.
Remove the onion, garlic, lemon slices and herbs. Save just the onion wedges and garlic for soups/stews.
Cooked lentils will keep in the fridge for around 5 days. Or, freeze for another time.