Combine the Fresh Ingredients: In a bowl, combine 1 small red onion, finely chopped, 2 tablespoons chopped cilantro, and 1-2 cloves garlic, crushed. Use a fork or a whisk to incorporate these fresh ingredients well, letting their flavors mingle.
Add the Creamy Base: Next, add 1 cup mayonnaise, 1 tablespoon lemon juice, 3 tablespoons harissa paste, 1.5 tablespoons tomato paste, 1 teaspoon ground cumin, and 1 teaspoon sugar to the bowl. Whisk the ingredients together until smooth and well-combined.
Taste and Adjust Seasoning: Carefully taste your sauce and adjust the seasoning. You can add more lemon juice for a tangier flavor or increase the harissa if you prefer more heat. Remember, the flavors will intensify as the sauce sits, so be mindful of how you adjust it!
Chill and Serve: For the best flavor, refrigerate the sauce for at least an hour. This waiting period allows the ingredients to meld beautifully together. Enjoy immediately or store in an airtight container in the fridge for later use.