Start by making the risotto. Preheat the oven to 180°C, 350°F, Gas 4.
Wash the Squash. Peel and remove the seeds then cut the flesh into 1cm/½in dice. Keep all the trimmings apart from seeds and skin. Arrange the remaining diced on a roasting tray, drizzle over a little olive oil and sprinkle over the garlic and rosemary. Roast them in the preheated oven until the squash is soft. Blend ½ the Butternut Squash and season (for the sauce).
Meanwhile, heat a little oil in a pan, add the shallot and leek and sweat until softened. Stir in the rice and season. Add half the wine, and slowly add the hot stock a ladleful at a time until the risotto is just cooked.
Add the fresh Peas, the other ½ roasted Butternut Squash and stir carefully. Fold in the butter, grated Parmesan, Mascarpone and Basil. Check the seasoning.
Heat a little olive oil in a small pan and sweat the Onion, Celery and Squash trimmings, then add the remaining white wine and cook until reduced by half.
Add the fish stock, reduce again by half, then add the cream, season and simmer for 5 minutes. Blend until smooth, season and stir in the Chives and chopped Tomatoes.
Cut the Artichokes in half lengthways and roast in the oven until golden brown.
Season the Halibut and pan-fry on both sides until golden brown. Transfer to a non-stick baking tray and cook in the oven for 5 minutes. Place a portion of Butternut squash purée on each place, using a metal ring mould to give a perfect shape then top with a portion of risotto. Place a Halibut fillet on top then arrange two artichoke halves around. Spoon the sauce around the Halibut and garnish with some crispy Leek and Chive cress.