In a medium, heavy saucepan over low heat, melt the butter. Whisk in the flour and cook, stirring often without allowing the flour to brown, for 4 minutes. Remove the pan from the heat and gradually whisk in the milk. Set the pan over medium heat. Whisk in the salt, pepper and nutmeg. Bring to a simmer, then lower the heat, partially cover and cook, stirring often, until thick, about 15 minutes. Remove from the heat. Add the Bel Paese/Gruyere, goat cheese, and Parmigiano-Reggiano and stir until smooth. The sauce can be prepared up to 3 days in advance. Cool, cover and refrigerate, returning it to room temperature before using.