Lay out the four slices of bread. Spread a thin layer of Dijon mustard on one side of each slice.

Assemble the sandwiches by layering one slice of cheese, half of the roast beef, and a second slice of cheese between two pieces of bread. Press the sandwiches down gently with your palm to compress the filling.

In a wide, shallow bowl, whisk together the eggs, milk, chopped thyme, salt, and pepper until well combined.

Melt the butter in a large non-stick skillet or griddle over medium heat.

Dip each sandwich into the egg mixture, allowing it to soak for about 10-15 seconds per side. Do not over-soak or the bread will fall apart.

Place the sandwiches immediately into the hot skillet. Cook for 3-4 minutes per side, or until the bread is golden brown and the cheese has fully melted. Adjust heat to low if the bread browns too quickly.

Remove from the pan, slice in half diagonally, and dust generously with powdered sugar. Serve immediately with raspberry preserves on the side.