Halve the avocado, remove the pit, scoop out the flesh, and place in a food processor. Add the yogurt and blend until smooth. Stir in the lemon zest and juice, dill, and garlic powder and set aside.
Peel the kohlrabi with a sharp knife. Grate the kohlrabi and onion with a box grater.
Wrap the grated kohlrabi and onion in a large clean dish towel, twist both ends, and squeeze tightly to remove any excess moisture and wilt the kohlrabi. Place the mixture in a large bowl. Add the eggs, salt, pepper, garlic powder and cayenne and mix with a large spoon until combined.
Stir in the arrowroot. If there is still liquid visible in the bowl, add more arrowroot to absorb and stir well to combine.
In a large skillet over medium-high heat, heat enough oil so that when melted, you have ¼ inch oil. Scoop out about ⅓ cup of the kohlrabi mixture and form into patties by hand. Fry for 2 to 3 minutes per side, until golden brown, and repeat until the entire mixture has been used. Season lightly with salt and serve topped withe the avocado-yogurt sauce.