Drain and rinse the garbanzo beans if using canned. Pat dry if very wet.
In a food processor combine the garbanzo beans, finely chopped onion, 1 egg, grated Gruyère, 4 tablespoons chopped parsley, 1 minced garlic clove, and 1 tablespoon olive oil. Pulse until you have a thick, slightly coarse paste.
Transfer the mixture to a mixing bowl, add 1/2 cup breadcrumbs, season with salt and pepper, and mix until combined. Let the mixture rest 10 minutes so the breadcrumbs absorb moisture.
While the mixture rests, make the tahini sauce: whisk together 2 tablespoons tahini, 2 tablespoons hot water, 1 tablespoon olive oil, 1 tablespoon lemon juice, 1 minced garlic clove, and salt and pepper to taste until smooth. Adjust thickness with more water if needed.
With slightly wet hands, portion the garbanzo mixture into 24 spoonfuls. Roll each into a ball and gently flatten into a croquette shape. Sprinkle or fold in the 2 tablespoons finely chopped Italian parsley as desired.
Heat 2 tablespoons olive oil in a large skillet over medium heat. Fry the croquettes about 2 minutes per side, until golden and crisp. Work in batches so the pan is not crowded.
Transfer cooked croquettes to paper towels to drain briefly, then serve warm with the tahini sauce.