In a blender or food processor, combine the tahini, lemon juice, cumin, garlic and 1/4 cup water; blend until smooth. Transfer to a bowl and stir in the tuna and parsley; season to taste with salt and pepper. Place a tomato slice on a plate, top with some tuna, another tomato slice and more tuna; sprinkle with parsley and sesame seeds. Repeat with the remaining ingredients.