Preheat the oven to 400°. Place a bell pepper on a sheet pan lined with parchment paper and roast for 25-30 minutes or until skin is blackened and papery.
Transfer the pepper to a small bowl and tightly wrap the bowl with plastic wrap. Let the pepper cool to room temperature. Peel the skins from the pepper and discard. Remove the stem and seeds and discard.
Transfer the flesh of the bell pepper to a mini prep food processor. Add the mayonnaise and minced garlic. Pulse until the pepper is fully blended into the mayonnaise, scraping the sides and pulsing again if necessary.
Transfer the aioli to a airtight container and refrigerate for several hours or overnight for the flavors to marry and the aioli to thicken.