In a large bowl or clay pot, add curd/yogurt, and whisk until smooth and creamy. You can also use a madani which is a traditional tool used to churn the curd.
Next, add powdered sugar, rose syrup, and water, whisk until well combined and frothy.
Add cardamom powder, if using, give it a good whisk.
Alternatively, you can simply use a blender to blend everything. Blending curd in the blender makes it a bit watery, so you may need just a couple of tablespoons of liquid to adjust the consistency.
Drizzle rose syrup on the sides of the serving glass to create a random pattern and pour the lassi. You may add a few ice cubes if you like.
Garnish with slivered pistachios and dried rose petals, and enjoy immediately!