Set a pan over medium heat. Pour in the milk and add the rosemary sprig. Bring to a simmer, stirring occasionally. Once it has come to a simmer, remove the pan from the heat and let it sit while preparing the roux.

Set a different saucepan over medium heat and add the butter to melt. Once melted, toss in the flour. Cook for 2-3 minutes while stirring, then remove from the heat.

Pour the hot milk into the roux. Vigorously mix together and scrape all the roux from the edges of the pan. Once combined, set the pan back over medium heat and bring to a simmer while continuing to stir constantly for 1-2 minutes. The sauce should begin to thicken.

• For the sandwich, I prefer it fairly thick so it’s almost spreadable. If it's too thin, continue to cook it down while stirring or make and add more roux. If it's too thick, add more milk a little at a time. Once at your desired consistency, remove from the heat.

Add flavorings and cheese: While still warm, add the comte, a spritz of lemon juice, and salt. Taste it! Dial in the flavorings exactly as you want them.

• To store, pour it into a bowl or container. To prevent a film from forming, drape plastic wrap so it rests on the top of the sauce, then move it to the fridge until ready to use.

Set the oven to broil.

Butter the bread and toast it on a pan. Spread a thin layer of mustard on the toasted bread, followed by some of the béchamel. Add the Emmental, then the ham, and half of the Gruyère. Place the second piece of bread on top and spread a thick layer of the béchamel over it. Lastly, add the remaining Gruyère on top.

Place the assembled sandwich on a sheet pan and slide beneath the broiler until melty and golden brown. Meanwhile, fry a sunny side egg in butter.

Pull the sandwich out of the oven, add the fried egg over top, and garnish with chives. Enjoy!