Add your cream cheese, sour cream, mayo, salt and mustard to a large bowl. Use a handheld mixer to beat until thoroughly mixed. Set aside (or place in the fridge, covered, if not using immediately). 

Trim the crusts off of your loaf of bread- this helps with even edges and even bread layers.

 For your first layer, place four slices of bread on your plate, two by two, so that they form a rectangle. (You can arrange these differently if you are trying to achieve a different shape with your cake).

 Spread a very thin layer of your cream cheese mixture "frosting" over the layer of bread. Alternatively, you can spread a thin layer of butter on the layers, and only frost the outside of the cake, if you prefer.

Carefully layer your first filling over the bread, being careful to avoid moving the bread slices. Even out the filling with an offset spatula if needed.

 Cover your filling with another layer of trimmed bread slices and repeat with the next filling.

Repeat once again with another layer of bread and a third filling. 

You can continue for as many layers as you see fit, especially if you are using very thin slices of bread- Note that you may need to double the "frosting" recipe if you are making a very large cake. When you have placed your top layer of bread slices on your cake, use the rest of the "frosting" to cover the entire cake.I tend to prefer a thin layer of the spread, so the bread will probably show through in a couple spots- that's okay, that's what your garnishes are for. 

Slice your garnish vegetables to your desired sizes and shapes, and place them on the top and sides of the sandwich cake in a way that appeals to you!