In a small saucepan, heat the milk just until it starts to bubble, then set aside.

In a large heavy bottomed skillet, melt the butter. Once the butter is melted sprinkle in the flour, constantly stirring, cook for about 2 minutes until bubbles start form, but don’t let it turn brown. Add in the hot milk while stirring until the sauce thickens, cook for about 3 minutes.

Remove from heat, toss in about ¾ cup of the shredded Gruyere cheese and the dried thyme, mixing well. Set aside (if making ahead, cover with wax paper or a small layer of milk to prevent the Bechamel (white sauce) from forming a skin).

Preheat your oven to 400 F. Line a baking sheet with parchment paper.

Lightly toast your bread slices in a toaster or in the oven for about 2 minutes.

Assemble on baking sheet: Spread mustard onto bottom bread slice, then continue layering your sandwich with 2 – 4 slices of the Ham, a generous handful of the cheese and the top slice of bread. Spread a layer of the Bechamel sauce over the top of the sandwich. Repeat and make accordingly to amount serving.

Place in oven, bake until the cheese is melted. Switch the oven from 400° F to broil and finish off by broiling your sandwiches for about 2 – 3 minutes or until the Bechamel sauce is bubbling and starting to brown.