Finely chop the parsley. Destem and wash the grapes. Peel the cucumber, quarter it lengthwise, and scoop out small balls using a melon baller. Place in a colander, sprinkle with salt, and let drain.
Peel the garlic cloves and quarter five of them, leaving one whole.
Pierce the monkfish tail at regular intervals.
Place in an ovenproof dish and dot with 30 g of Guérande sea butter in small nuggets.
Preheat the oven to 240°C (gas mark 8).
Roast the monkfish for 15 minutes, then reduce the temperature to 180°C (gas mark 6) and cook for another 15 minutes. Baste regularly and turn halfway through.
In a pan, sauté the cucumber balls for 5 minutes in the remaining 20 g of sea butter. Add the minced garlic clove and grapes, then cook for another 5 minutes.
Combine the chopped parsley with the verjuice, 2 tablespoons of cream, and a pinch of salt. Transfer to a heavy-bottomed enamel saucepan, bring to a brief boil, then stir in the remaining tablespoon of cream off the heat. Season lightly with salt and pepper.
Slice the roasted monkfish and serve with the grape-cucumber garnish and parsley-verjuice cream.
Verjuice: This tangy cousin of vinegar is the unfermented juice of unripe grapes or sour apples. Once a staple in Southwest French cuisine, it was revived by chef Bernard Loiseau.
