Note: I love to use crispy ham in this recipe, but you don’t have to.
Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
Heat a large nonstick skillet over medium-high heat. Once hot, add ham in batches, turning with kitchen tongs and cooking until crispy on all sides. Remove and place on a plate. Once cool, tear into pieces since the crostini is smaller.
Heat a small saucepan over medium heat and add the butter. Once sizzling, whisk in the flour and cook until golden, about 1 to 2 minutes, to create a roux. Add milk and stir constantly until slightly thickened, then reduce heat to low and add in gruyere. Stir until melted, then season with salt, pepper and nutmeg.
In a bowl, mix together the grated gruyere and parmesan.
Take each slice of baguette and spread it with the grainy mustard. Add a sprinkling of cheese, then top that with a piece of crispy ham. Cover the top with a heaping spoonful of cheese sauce. Add another sprinkling of grated cheese. Repeat with all the bread slices.
Bake the crostini for 15 minutes, until golden and bubbly on top. If desired, you can broil at the end for a moment to make the cheese even more bubbly and golden.