Mix the sesame oil, soy sauce, Sriracha and mayo together and spread on one side of each piece of toast. Add the sliced tomato and spring onion on top of one slice of the bread, on the side with the Sriracha mayo, you can add the salt if you would like. Add 2 slices of vegan cheese on top on the tomato and add the other piece of bread, Sriracha mayo side down to make a sandwich.