6medium-size plum, Roma or Campari tomatoes, halved lengthwise
1small sweet onion, in quarters
3tablespoonsolive oil, divided
2teaspoonskosher salt, divided
1 1/4teaspoonsblack pepper, divided
1 1/2poundsFlank steak
1tablespoonbalsamic or red wine vinegar
1garlic clove, finely chopped
1/2teaspoongranulated sugar
Instructions
Preheat grill to medium-high (about 450°F), or heat a grill pan over medium-high.
Toss together tomatoes, onion, 1 tablespoon of the oil, 1/2 teaspoon of the salt, and 1/4 teaspoon of the black pepper in a medium bowl; set aside.
Rub Flank steak with 1 tablespoon of the oil and remaining 1 1/2 teaspoons salt and 1 teaspoon black pepper.
Grill tomatoes and onion turning often, until charred and softened, 4 to 6 minutes. Remove from grill, and let cool until ready to use.
Grill steak, turning occasionally, until lightly charred and medium-rare, 8 to 10 minutes. Transfer to a board, and let rest, 5 to 10 minutes.
Coarsely chop tomatoes and onion, and transfer to a medium bowl. Add vinegar, garlic, sugar, and remaining 1 tablespoon olive oil, and toss to combine.
Slice steak against the grain, and spoon tomato relish over top. Serve immediately.