1. Make the sumac onions: In a bowl, stir together all ingredients. Set aside for at least 30 minutes.
2. Make the tzatziki: Using the largest holes on a box grater, shred cucumber. Transfer to a fine mesh strainer to drain water from cucumber. Meanwhile, grate garlic into yogurt and stir to combine. Stir in remaining ingredients and drained cucumber.
3. Make the sausage: In a large bowl, combine all ingredients except feta. Mix until well-combined. Gently fold in 1 ounce feta. Hand-form into sausage shapes and grill until internal temperature reaches 165 degrees.
4. To assemble: Spread hot dog bun with tzatziki. Add sausage and top with sumac onions, tomatoes, and cucumbers. Top with crumbled feta.