Pre-heat your broiler or toaster oven to 350 degrees. Arrange mussels on a rimmed baking sheet. Avoid using a rimless sheet if you can because the mussels will release liquid as they cook. Make the spicy mayo. To prepare the sauce, mix the mayo and the Sriracha sauce in a bowl until the mixture is smooth. Do a taste test here. If you would like the sauce to be hotter, add a little more Sriracha. If you want it to be milder, add more mayo. The consistency of the sauce should be like pancake batter or a softened milk shake. Top the mussels. Once you’ve gotten the mayo to your liking, spoon about a tablespoon of sauce over each mussel. Put just enough to cover the meat completely, about 1 tablespoon per mussel. Bake the mussels. Plate the mussels in your preheated oven for about 10 minutes. Check frequently and rotate the pan occasionally if you need to to even out the browning. Cook until the sauce bubbles and gets golden brown with a few dark spots forming. The total cooking time should not exceed 15 minutes. Serve immediately. Be sure to provide an empty bowl for the discarded shells.