Preheat the grill. Arrange the halibut fillets on the grill rack, placing a knob of butter on top of each one. Sprinkle them with a few drops of lemon juice, then season with a little salt and pepper. Grill for 6 to 8 minutes, depending on the thickness of the fillets.
While the fish is cooking, put the rest of the butter into a saucepan with the lemon zest, remaining lemon juice and mustard. Heat gently, whisking until melted, then add the thyme sprigs. Keep warm over a low heat.
Heat the olive oil in a wok or large frying pan and sauté the leeks and courgettes for 4 or 5 minutes. Add the spinach and wine or stock, stirring until the leaves have wilted – about 1 minute.
Serve the fish and vegetables, pouring a little warm lemon and thyme dressing over each one.
Cook’s tip: Use regular thyme if you can’t find the lemon variety, or use rosemary instead.
• 4 x 100-150g (4-6oz) fillets wild Alaska halibut
• 75g (3oz) butter
• Finely grated zest and juice of 1 large lemon
• 2 tsp Dijon mustard
• 4 sprigs lemon thyme
• 2 tbsp olive oil
• 2 leeks, sliced
• 2 courgettes, sliced
• 225g (8oz) baby spinach leaves
• 2 tbsp dry white wine or vegetable stock
• Salt and freshly ground black pepper