Tuna is one of the best fish to choose for your grill. Here it works to unite the sweet-tart tropical fruit juice with the heat of the chiles. Any kind of hot pepper, such as jalapeño or serrano, will work in place of the habanero.
Combine first 10 ingredients in a large saucepan. Bring to a boil, and cook until reduced to 1 1/2 cups (about 15 minutes); remove from heat. Combine cornstarch and water in a small bowl; stir well with a whisk. Add to pan. Bring to a boil; cook 1 minute, stirring constantly. Remove from heat; stir in onions.
Place tuna on grill rack coated with cooking spray; grill 3 minutes on each side or until medium-rare or desired degree of doneness, basting frequently with glaze.