Melt the butter in a heavy saucepan over medium heat. Stir in flour until smooth, then continue to cook and stir until lightly browned, about 10 minutes. Gradually stir in the milk so that no lumps form and then simmer until thick, stirring the whole time. Remove from the heat.Advertisement
Stir together the baking soda and canned tomatoes; stir into the sauce. Whisk together the mustard and eggs; stir into the tomato sauce. Set the saucepan over a pot of barely simmering water (like a double boiler). Stir in the cheese and season with salt and pepper. Continue to cook, stirring occasionally, until cheese has melted.