Mix mayonnaise, sriracha, lime juice, salt and agave, if using, till it becomes a smooth mix. Check the taste and adjust to your liking. Refrigerate the sauce till ready to use. The sauce can be made 3-4 days ahead of time.
Use sriracha or your favorite chile sauce. I love the flavor and heat level of Lee Kum Kee Sriracha, but use your preferred brand (some are spicier than others). Alternatively, sambal oelek, Cholula, Tapatio, Louisiana's Tabasco sauce or any hot sauce you have on hand will do.
Customize to your liking: This ratio of mayo-to-sriracha creates a sauce that is medium-spice level. Add more or less depending on if you like it mild or spicy. To make it spicier, add 1-2 teaspoons more of sriracha. Make it mild by reducing the sriracha in half. To make it sweeter, double the agave or honey.
Serve immediately or make-ahead. The spicy mayo can be served right upon mixing, but the flavors get better the longer they sit together. If adding minced garlic or onion (as in the variations above), prep at least a few hours ahead of time to take the raw bite away from the aromatics.
Store in an airtight container in the fridge. The sauce will keep for up to 4 days.
Note: The nutrition facts below are my estimates and are based on 1 tablespoon serving size. If you are following any diet plan, I recommend cross-checking with your preferred nutrition calculator.