BRITTLE
Preheat oven to 350°. Bring sugar, cardamom, and ¼ cup water to a boil in a small saucepan, stirring to dissolve sugar; let cardamom syrup cool.
Lightly coat a parchment-lined baking sheet with nonstick spray.
Toss pistachios and sesame seeds in a small bowl with half of syrup and spread out on prepared baking sheet.
Bake, tossing occasionally, until golden, 12–15 minutes; season with salt. Let cool (brittle will harden as it cools), then break into small pieces.
ASSEMBLY
Combine lemon juice, rose water, and remaining cardamom syrup in a small bowl.
Spoon yogurt onto plates.
Top with melon and brittle, drizzle with lemon-cardamom syrup, and season with salt and pepper.