Put a large, ovenproof casserole dish on a medium-low heat
Add the oil, then the onion and cook, stirring regularly, for six to eight minutes until it begins to soften
Add the garlic, rosemary and some salt and pepper. Cook gently for a further five minutes
Stir in the lentils and stock
Season the chicken thighs and place skin-side up in the casserole. You want most of the chicken skin to remain exposed above the liquid in the dish so it can brown in the oven
Bring to a simmer on the hob, then transfer to the oven and bake, uncovered, for one hour
Check that the chicken is cooked right through and the lentils are soft. If not, return to the oven for 10-15 minutes and test again.
Skim off any excess fat from the surface. Taste the lentilly liquor and add more salt or pepper if needed. Serve, scattered with chopped parsley, just as it is or with steamed broccoli or spring greens on the side.