1. Pre-heat your broiler/grill to 180° C / 350° F. To prepare the sauce, first drop a pinch of the Hon Dashi pellets in a medium bowl and dissolve completely with the half and half.
2. Add the Kewpie mayo. Combine the mixture with a spoon until smooth. Add the Sriracha and fully incorporate into the sauce.
3. Taste test here. For a hotter sauce, add a little more Sriracha. For a milder sauce, add a few squirts of Kewpie mayo. If the sauce is too thick, thin the mixture slightly with a few drops of half and half. The consistency and viscosity of the sauce should be like pancake batter or a softened milk shake.
4. Add the masago and stir slowly to distribute evenly into the sauce. Spoon the sauce over each mussel. Allow just enough to cover the meat completely. Place the mussels under the broiler/grill to cook. Check frequently and rotate the pan occasionally to even out the browning and compensate for hot spots.
5. Cook until the sauce bubbles and turns golden brown with a few dark spots forming. The total cooking time should not exceed 15 minutes. Serve immediately.