Toss onion, tomatoes, bell pepper, and chile with oil in a shallow (1-inch) baking pan and arrange vegetables, cut sides down, in 1 layer. Roast, turning onion once or twice, until vegetables are charred and tender, about 20 to 25 minutes.
Discard skins from tomatoes and bell pepper. Chop tomatoes and drain in a sieve, discarding juices. Finely chop onion and bell pepper. Mince chile.
Mince garlic and mash to a paste with kosher salt. In a bowl, blend together mayonnaise, garlic paste, and black pepper. Add chile, about 1/4 at a time, tasting for desired heat. Stir in tomatoes, bell pepper, onion, and vinegar.
Aioli can be made 1 day ahead and chilled, covered.