Spread 2 tablespoons of mustard on one side of enough slices of rye bread to cover the bottom of the baking pan. Repeat on an equal number of rye slices to form a top layer. Place first layer of bread in the pan, mustard-side up. Cover the layer of bread with ham, cheese, tomato (reserve 2 tablespoons for garnish), and onion. Place remaining bread mustard-side down on the layers. Beat together eggs, 2½ cups milk, and ground mustard. Pour this over the ingredients layered in the pan. Place in oven and bake 1 hour or until set. Let the casserole cool while making the sauce.
For sauce, melt butter in 2 quart saucepan. Add flour and blend well. Gradually add 2½ cups milk and heat until thickened and bubbly. Add 2½ cups spinach and stir for 2 minutes. Serve on plates sprinkled with remaining spinach. Ladle desired amount of sauce over each portion and garnish with chopped tomato.