2½ pounds ripe heirloom tomatoes (about 4 beefsteak)
About 2½ tablespoons (3 envelopes) unflavored gelatin
¼ cup very hot water
2 teaspoons balsamic vinegar
1½ teaspoons kosher salt
1 teaspoon chopped thyme leaves
¼ teaspoon crushed red pepper flakes
Lightly grease eight 4-ounce ramekins with cooking oil spray.
Seed one of the tomatoes, letting the seeds, gel and liquid fall into the bowl of the food processor. Finely dice the flesh to yield ½ cup; transfer to a small bowl. Cut the remainder of the seeded tomato and the remaining whole tomatoes into 1-inch chunks and add to the food processor. Process until completely pureed, about 1 minute. Strain into a 1-quart liquid measuring cup through a fine-mesh strainer, using a flexible spatula to press through as much of the juice as possible; discard any solids in the strainer. You'll need 2¾ cups of juice; reserve any extra for another use (such as a gazpacho or vinaigrette). If you don't have enough juice, add water to yield a total of 2¾cups.
Transfer a generous ½cup of the juice to a very wide, shallow bowl. (You need a large surface area.) Sprinkle the gelatin evenly onto the juice, letting it sit on the surface. After 10 minutes, add the hottest possible tap water and stir until the gelatin has dissolved and the mixture is thick yet not lumpy. Add it to the remaining 2¼ cups of tomato juice in the measuring cup, stirring to incorporate completely. Add the reserved ½cup of diced tomato, along with the vinegar, salt, thyme and crushed red pepper flakes.
Pour equal amounts of the aspic mixture into each ramekin. Cover with plastic wrap and refrigerate until the aspic sets, at least 4 hours and up to 2 days.