butcher's twine, a meat thermometer and metal skewers
Prepare an outdoor grill for medium-high heat, leaving 1 burner off for indirect heat (to finish the roulades). Oil the grill grates.
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 set aside.
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. Remove the plastic wrap and 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. Starting with the long side 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) at each end and at 2-inch intervals in between. Brush with oil and sprinkle with salt and pepper.
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 meat 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.
Meanwhile, lay a slice of onion 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 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.
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