In a bowl, combine your Tuna, Mayo, Mustard, Onion, Parsley, Garlic, Seasoning, Cayenne Pepper and Lemon Juice. Place to one side.
Half, destone and slice your Avocado into evenly sized strips (8 strips per half). Half your Tomatoes and slice to similar thickness. You should end up with 16 slices of Avocado and 16 slices of Tomato.
Evenly slice your bagels in half and toast either under the grill or in toaster.