1½ lb. tomatoes, cored 1 habanero chile, stemmed 1 tsp. kosher salt 1 clove garlic, smashed 2 tbsp. olive oil ½ small white onion, minced
1. Heat oven to 500°. Place tomatoes and chile on a foil-lined baking sheet, and roast, without turning, until chile is browned in spots, about 10 minutes; transfer chile to a blender, and continue roasting tomatoes until blackened and cooked through, about 20 minutes more. Let tomatoes cool and then peel and place in blender along with salt and garlic; puree until smooth.
2. Heat oil in a 4-qt. saucepan over medium heat; add onions and cook, stirring, until soft, about 5 minutes. Add purée, and bring to a simmer; cook, uncovered and stirring often, about 10 minutes. Let cool.