Preheat the oven to 350 degrees . Brush the interior of a 6-cup soufflé mold with the softened butter. Dust the bottom and sides with flour. Set aside.
In a heavy saucepan melt the butter. Stir in the flour with a wooden spoon and cook over low heat for 3 minutes. Add the milk. Cook over medium-high heat while whisking until sauce becomes thick and bubbly. Transfer the sauce to a large mixing bowl. Season with salt and pepper and nutmeg.
Separate the eggs. Mix egg yolks and cheeses in the thick cream sauce. (can be prepared 2 hours in advance up to this point.)
Stewed tomatoes:
In a medium saucepan, heat oil and shallots. Stir over medium heat for 5 minutes, Do not brown. Add tomatoes, and simmer until sauce is thick .
In an electric mixing bowl, whip the egg whites with cream of tartar until the whites are firm and fluffy. Using a rubber spatula, gently fold the whites into the cheese mixture. Pour one half of the mixture into mold. Spoon stewed tomatoes over soufflé and add remaining soufflé mixture. The mold will be full. Bake for 45 minutes. Take the soufflé out of the oven and allow to settle for 20 minutes on a large plate. Invert the soufflé over a serving platter.