In a medium bowl, whisk together evoo, ¼ cup lemon juice, garlic, sumac, parsley, and shallots.
Add fish to marinade, cover, and marinate for at least 30 minutes but no longer than 1 hour. (Fish do not have connective tissue and cannot be marinated longer or the fish will cure and “cook” with the acid in the marinade.)
Skewer fish on soaked bamboo skewers.
Tahini
In a medium bowl mix tahini with remaining ¼ cup lemon juice and ¼ cup water. Stir and add more water to reach desired consistency.
Preheat grill to medium-high. Soak an old towel in evoo and wipe down grill grates.
Grill fish kabobs for about 3 minutes per side.
Transfer grilled kabobs to a platter and drizzle with tahini. Garnish with sumac, fresh parsley, and schug.