Prepare the spice rub for the salmon fillets by combining all the ground spices with the lemon juice and seasoning in a small mixing bowl until smooth. Rub the salmon fillets with the spice rub, arrange on a baking tray lined with nonstick baking paper. Cover and chill.
Combine the lemon juice, shallot and black peppercorns in small saucepan. Reduce the mixture by half over a moderate heat. Remove from the heat and leave to infuse to one side.
Preheat the oven to 220°C.
Remove the salmon escalopes from the fridge and leave at room temperature for 10 minutes.
Strain the reduced lemon juice into a clean saucepan. Heat over a medium heat and whisk in cubes of butter, one at a time, until you have a thickened sauce. Add the cream once you have whisked in all the butter. Season to taste and keep over a very low heat, stirring occasionally.
Roast the salmon in the oven for 8-10 minutes.
Meanwhile, heat the olive oil in a large frying pan over a moderate heat and sauté the spinach, with some salt for 2-3 minutes until wilted. Spoon the wilted spinach onto serving plates and rest the roasted salmon fillets on the spinach. Spoon some of the sauce around the plate. Garnish with a pinch of curry powder and a slice of lemon before serving.