Prepare the pickled red onions* ahead of time. They need at least 4 hours to pickle but preferably overnight.
For the Brussels Sprout Filling
Once you've shredded the Brussels sprouts and minced the garlic, heat the olive oil in a large sauté pan over medium-high heat for just a minute.
Add the shredded Brussels sprouts, garlic, chili powder, paprika, and a pinch or two of salt.
Sauté for 5 to 10 minutes, or until softened to desired texture (I like a bit of bite left in mine), stirring every minute or two.
Taste and add more sea salt, if desired.
For the Sriracha-Lime Mayo
Meanwhile, in a medium mixing bowl or jar, whisk together the mayo, Sriracha, and lime juice.
Keep refrigerated until ready to serve.