Combine Greek yogurt, buttermilk, avocado, parsley, dill, chives, onion powder, garlic, salt, and pepper in a blender.
Blend until the mixture is smooth and creamy. Adjust seasoning if necessary.
Cover the dip and refrigerate for at least 30 minutes to allow flavors to meld.
Heat vegetable oil in a pot to 375°F (190°C) for frying.
Prepare a batter by whisking flour, baking powder, and cayenne pepper in a bowl.
Gradually add sparkling water to the dry ingredients, stirring until just combined to form a lumpy batter.
Dip asparagus and zucchini slices into the batter, ensuring they are fully coated.
Fry the battered vegetables in the hot oil for 60-70 seconds until golden and crispy, then drain on paper towels and season with salt.