Melt the butter in a skillet over medium high heat. Add the onions and cook, stirring frequently, until the onions are translucent, about 4 minutes. Add the garlic, Herbal Pizza & Pasta, Seafood Magic and salt. Stir well and lower the heat. Cook until the seasonings have darkened slightly, about 2 minutes. Add the flour. Stir until the white of the flour has disappeared. Add the stock and bring to a boil, stirring frequently until the sauce thickens, about 2 minutes. Add the cream and return to a boil, stirring constantly. Remove from heat and chill.
When cold, add the cheeses and stir to distribute evenly.