For the pipian
Take the fish out of the fridge. Warm an old frying pan over a medium heat and toast the pinenuts on all sides for a few minutes until pale golden. Transfer to an upright food blender and then toast the pumpkin seeds until golden, puffed up and beginning to pop. Separate out the 30g for garnish and empt the rest into the same blender. Now peel the onion, cut into wedges and add to the pan with the garlic cloves, which you have left unpeeled, and the jalapeno chilli. Turn the heat up and dry-roast for 7-8 minutes until the onion wedges are blackened, the garlic is soft and the jalapeno is blackened and also soft. Slip the garlic cloves from their skins and add to the blender with the onions, chilli and rest of the ingredients.
Start the motor running and slowly pour in 200ml warm water and a teaspoon of sea salt and blend for 3-4 minutes until the mixture is completely smooth and is about the consistency of double cream. Heat a pan over a medium-high heat and pour in half the oil. Pour in the pipian and stir for a minute - be careful, it will splutter a little - before pouring in another 200ml water. Now taste the sauce and season with a little more salt if its needed.
Pre-heat the oven to 200C. Warm the coriander seeds in a small frying pan and crush with the garam masala and ½ teaspoon of sea salt. Rub the fish with 2 tbsp olive oil and the spices. Slice the lemon and put the slices inside the fish cavity, season it with salt and pepper. Bake the fish for 15 minutes, or until a skewer can be pushed lightly and easily through the thickest part of the fish with no resistance.
Re-heat the sauce and taste, adjust the seasoning. Cut the fish down the middle and then across the fillets. Spoon a pool of sauce onto each warm plate, followed by the fish. Top with a pile of watercress leaves, a sprinkle of pumpkin seeds and a wedge of lime. I love this with rice and greens, but it is also delicious with oven-roasted potatoes.
