1. Roast jalapeno over a gas flame or under a broiler until blackened. Let cool in a paper bag. Peel, seed, and cut jalapeno into quarter-inch pieces.

2. Remove peel and pith from all citrus using a sharp paring knife. Place a strainer over medium bowl. Working over a medium bowl to catch the juice, carefully cut between membranes to remove segments. Squeeze remaining membrane to extract juice; reserve. Cut segments into 1/4-inch pieces; transfer to a separate bowl.

3. Add onion to citrus. Halve tomatoes lengthwise, squeeze out seeds; cut into 1/4-inch pieces. Gradually add reserved peppers, tasting for heat. Stir in thyme and oil, adding reserved citrus juice if salsa is dry.