Place quinoa and 2 cups of water in a small saucepan. Bring to a boil over high heat. Cover, reduce heat and simmer for 15 minutes. Uncover and remove from heat. Drain tuna and place oil in a large bowl. Add lemon juice, ¼ cup water and mustard, whisking to combine. Add quinoa to oil mixture, tossing to combine. Stir in onion, celery and bell pepper. Season with salt and pepper. Stir in parsley and mint before serving.