How to Make It
In a large skillet, combine the cumin, coriander, mustard and fennel seeds with the cardamom and pink and white peppercorns and toast over moderate heat, shaking the pan, until fragrant, about 2 minutes. Transfer to a spice grinder and let cool. Add the fenugreek, paprika and cayenne and finely grind the spices.
In a food processor, combine the ground spices with the garlic and shallots and process until fine. With the machine on, add the olive oil and process until a thin paste forms. Return the mixture to the skillet and warm over low heat, about 10 minutes. Transfer the curry oil to a large jar and refrigerate for 2 days.
Line a sieve with a double layer of cheesecloth and set it over a bowl. Add the curry oil and strain, pressing on the solids to extract as much oil as possible. You should have about 2/3 cup. Discard the spices.
Fill a soup pot with 1/2 inch of water and bring to a boil. Add the spinach and cook, tossing, until wilted, about 2 minutes for red spinach and about 5 minutes for green. Drain the spinach and press it dry, then coarsely chop. Wipe the pot dry.
Add 1/4 cup of the strained curry oil to the pot and heat until shimmering. Add the spinach, season with salt and cook over moderately high heat, tossing, just until heated through, about 5 minutes. Transfer the spinach to a bowl and serve.