Put the lentils, bay, garlic, celery and a pinch of salt in a pan, cover with one and a half litres of water, and bring to a boil. Turn down the heat to a simmer and cook for 30 minutes, until the lentils are tender but still hold their shape.
While the lentils are cooking, put the grapes on a small baking tray, pour over some oil, sprinkle with salt, add the marjoram, and toss.
Bake at 200C (180C fan)/390F/gas 6 for 20 minutes, until soft and blistered, then pull from oven, put the cheese on top and return to the oven for five minutes, until the cheese is warm.
Drain the lentils, then put them in a bowl and toss with three tablespoons olive oil, salt, lots of black pepper, the red-wine vinegar and parsley. Share the lentils between four plates and top each serving with some of the grapes and a cheese.