Preheat oven to 350°F. Toast walnuts on a rimmed baking sheet, tossing occasionally, until golden brown, 8–10 minutes. Let cool, then finely chop. Mix walnuts, cucumber, yogurt, raisins, green garlic, lemon zest, lemon juice, 1 Tbsp. mint, 1 Tbsp. oil, and 2 tsp. rose petals, if using, in a medium bowl to combine; season with salt and pepper. If yogurt looks thick, stir in milk as needed. Serve topped with Aleppo-style pepper and more dried mint and rose petals and drizzled with more oil. Yogurt can be made 1 day ahead. Keep chilled.