Preheat oven to 475°. Combine tahini, capers, olive oil, and 3/4 teaspoon crushed red pepper in a large bowl, stirring with a whisk. Add cauliflower florets; toss to coat. Arrange cauliflower in a single layer on a foil-lined baking sheet coated with cooking spray. Roast 15 minutes, stirring halfway through cooking. Remove from oven; squeeze juice of lemon over top. Sprinkle with parsley, salt, and, if desired, additional red pepper.