Start by melting the butter in a saucepan over medium heat. Once melted, sprinkle in the flour to create a roux. Whisk constantly for about two minutes, keeping things moving to avoid burning.

Gradually pour in the warm milk, continuing to whisk. You want to avoid lumps here. Keep whisking until the mixture thickens, about four to five minutes.

Once your sauce has thickened, lower the heat and stir in the Gruyère and Parmesan cheese. Keep stirring until everything is melted and smooth. Don’t forget to season with salt and pepper, and toss in that nutmeg and whole grain mustard if you’re feeling adventurous. Set this aside.

Grab your slices of bread. Spread Dijon mustard on each piece. Layer with slices of ham, and then coat them generously with the Mornay sauce.

Preheat your oven to 400°F (200°C). Place your assembled tartines on a baking sheet. If you’ve got any leftover Mornay sauce, drizzle it over the tops for extra flavor.

Pop the baking sheet into the oven for about 10–15 minutes. You want the edges to get nice and golden, while the cheese bubbles and creates that irresistible crust.

While the tartines bake, find a non-stick skillet. Prepare your eggs to your liking. I prefer mine sunny-side-up or slightly runny—this way, the yolk cascades over the tartine when you cut into it.

Once the tartines are golden and bubbly, take them out of the oven. Top each one with the perfectly cooked egg. Optionally, garnish with fresh herbs like parsley or chives. Dig in and enjoy!