2 tablespoons olive oil
1 6-inch rosemary sprig
½ cup plain whole-milk Greek yogurt
1 tablespoon honey
1 3-pound seedless watermelon, rind removed, cut into 2-inch pieces
3 tablespoons fresh grapefruit juice
Flaky sea salt
1 teaspoon crushed red pepper flakes
1 teaspoon poppy seeds
Heat oil in a small skillet over medium. Fry rosemary until crisp, about 20 seconds. Transfer to a paper towel to drain; set rosemary oil aside.
Mix yogurt and honey in a small bowl; season with kosher salt.
Toss watermelon and grapefruit juice in a large bowl; season with flaky sea salt.
Spread yogurt on a large platter. Top with watermelon and crumble fried rosemary over. Drizzle 1 Tbsp. reserved rosemary oil over (save remaining oil for another use) and sprinkle with red pepper flakes and poppy seeds.