1 (2- to 2 1/4-lb) tri-tip beef roast (also called triangular roast; about 2 inches thick)
1 1/2 teaspoons salt
1/4 teaspoon black pepper
1 tablespoon olive oil
1 1/2 lb cherry tomatoes (5 cups)
1/2 teaspoon salt
1/4 teaspoon dried hot red-pepper flakes
1/4 cup plus 2 tablespoons extra-virgin olive oil
1 cup firmly packed fresh flat-leaf parsley leaves
1 garlic clove, sliced
1 tablespoon red-wine vinegar
Special equipment: a well-seasoned 10-inch cast-iron skillet; an instant-read thermometer