Peel and slice boiled potatoes, and put them into an ovenproof dish. Wash fresh rosemary and dry it. Chop the rosemary needles and scatter them over the potatoes. Add salt and pepper.
Wash the broccoli and cut it into chunks. Cook with salt in a very small quantity of boiling water until firm to the bite.
Take broccoli out of the water and spread it over the potatoes. Whisk the broccoli water with egg, verjuice and cream and pour the mixture over the vegetables. Coat with grated cheese.
Put into the oven and heat from above for about 10 minutes.