In a large bowl, combine the rice, tuna, chickpeas, cucumbers, tomatoes, and red onions.
Add the chopped parsley and sprinkle with sumac, if using.
Drizzle the olive oil and lemon juice over the mixture, then season with salt and pepper.
Toss gently to combine everything well.
Top each bowl with tahini sauce or a spoonful of Greek yogurt if desired. Serve immediately.