Tear off an 18-inch-long piece of heavy-duty foil. Toss 2 cups unsalted roasted mixed nuts with 2 tablespoons olive oil, 1 rosemary sprig, 2 wide strips lemon zest, 2 crushed garlic cloves, 1 teaspoon kosher salt and a pinch of red pepper flakes on the foil. Bring the two short ends of the foil together and fold to seal, then fold in the sides. Grill over medium heat, flipping occasionally, until the nuts are hot and toasted, 8 to 10 minutes. Carefully open and sprinkle with flaky salt.