Peel and coarsely chop 1 medium yellow onion, 3 large garlic cloves, and 1-inch piece fresh ginger. Finely grate the zest of 1 small lemon. Juice the lemon until you have 1 tablespoon juice. Finely chop 5 large anchovies.

Heat 1 tablespoon anchovy oil in a medium saucepan over medium-high heat until shimmering. Add the onion, garlic, and ginger, and cook, stirring occasionally, until caramelized and slightly softened, about 8 minutes. Add the anchovies and ¼ cup tomato paste, and cook, stirring occasionally, until anchovies are fragrant, about 1 minute.

Add 1 ¼ cups malt or apple cider vinegar, ¾ cup water, ½ cup raisins, ⅓ cup granulated sugar, ⅓ cup molasses, ⅓ cup tamarind concentrate, 1 tablespoon celery salt, 1 teaspoon ground cloves, ¾ teaspoon ground black pepper, ¾ teaspoon curry powder, and ¼ teaspoon cayenne pepper if using. Bring to a simmer over medium-low heat. Simmer, stirring occasionally, until well combined and very fragrant, 15 minutes. Remove the saucepan from the heat. Add the lemon zest and stir to combine. Let sit for 10 minutes to cool slightly and let the flavors meld.

Transfer the sauce to a blender and purée until smooth. Pour through a fine-mesh strainer set over a medium bowl or jug, pressing and stirring with a rubber spatula to push the liquid through. This may take up to 10 minutes, as the sauce is very thick at this stage. Discard the solids in the strainer.

Add the lemon juice and stir to combine. Taste and season with more celery salt as needed. If you feel the sauce is a little too thick, whisk in 1 to 2 tablespoons water to loosen.