Special equipment: Butcher's twine, meat thermometer, metal skewers
Heat the oil in a large skillet over medium-high heat. Add the broccoli, garlic and 1/4 teaspoon salt, and cook, stirring, until the broccoli begins to brown in spots, 3 to 4 minutes. Add 1 cup water, and continue to cook until the water evaporates and the broccoli is tender, 3 to 4 minutes more. Transfer to a bowl, stir in the Cheddar and reserve.
Butterfly the tenderloins by making a 1-inch-deep incision down the length of each without cutting all the way through. Open the tenderloins like books so they lie flat, cover with plastic wrap and pound with the flat side of a meat mallet until they're about 1/4 inch thick. Sprinkle with salt and pepper. Spread the broccoli mixture over the 2 tenderloins, leaving a 1-inch border around all the sides except the long one closest to you. Tightly roll up each tenderloin, tucking the narrow ends inside as you roll. Tie each roulade with butcher's twine tightly (without piercing the meat) on each end and at 2-inch intervals in between. Brush with oil, and sprinkle with salt and pepper.
Prepare an outdoor grill for medium-high heat, leaving 1 burner off for indirect heat (to finish the roulades). Oil the grill grates. Grill the roulades over direct heat, turning, until the outside is well marked, about 12 minutes. Move them to indirect heat, cover and cook until a thermometer inserted in the center of each roulade registers 145 degrees F, about 15 minutes more. Transfer them to a cutting board to rest 10 minutes.
While the roulades are cooking, make the relish: Lay an onion slice on a cutting board. Place the palm of 1 hand on top of the slice, and with the other hand push a metal skewer through the slice crosswise until it comes out the other side. Repeat with the other slices, leaving an inch of skewer uncovered on each end and using as many skewers as necessary. Thread the tomatoes on the skewers. Brush the onions and tomatoes with oil. Grill, turning, until the tomatoes are charred and bursting and the onion slices are tender with brown edges, 6 to 7 minutes. Remove the tomatoes and onions from the skewers, and chop. Mix with the parsley, vinegar, 1 teaspoon salt and a few grinds of pepper in a medium bowl.
Cut the strings off the roulades with scissors, and slice the roulades on the bias. Serve with the relish.