Preparation Method
- In a small saucepan over medium-high heat, melt butter. Add flour, and cook, whisking, until smooth, about 1 minute. Whisk in milk, and bring to a boil. Reduce heat to maintain a steady simmer, and cook, stirring occasionally, until béchamel sauce is reduced and thickened, 8 to 10 minutes. Stir in one-third of Gruyère, Parmigiano-Reggiano, and egg yolks into béchamel sauce. Season with salt, pepper, and nutmeg.
- Spread mustard on one side of 4 slices of bread and top with ham slices and remaining Gruyère. Top each sandwich with a remaining piece of bread.
- Heat the griddle to 350°F (180°C). Add sandwiches to the griddle. Cook, flipping once, until golden brown and crusty on both sides, about 10 minutes. Using a rubber spatula, lift up each sandwich and pour one-fourth of the béchamel sauce on the griddle underneath each sandwich. Lower sandwich back onto the pool of sauce. Cover the griddle, and cook until béchamel sauce sets into a soft, frico-like cheese crisp, about 3 minutes. Repeat process for other side of each sandwich. Invert sandwiches onto serving plates, and serve while hot.
Variations
MOZZARELLA B.A.T.
Make as directed. Substitute fresh mozzarella for Gruyère, cayenne for nutmeg and cooked slices of bacon for ham slices. Add ¼ cup (5 grams) packed arugula and 2 slices tomato (30 grams) inside each sandwich before cooking.
WELSH RABBIT TARTINE
Make as directed, omitting nutmeg. Substitute 1 cup (250 milliliters) brown ale for 1 cup (227 grams) milk and white Cheddar cheese for Gruyère. Add 2 tablespoons (30 milliliters) Worcestershire sauce to béchamel sauce. Reduce to 4 slices sourdough bread, cut 1-inch thick. Using a paring knife, cut a horizontal slit halfway through one long side of each bread slice and stuff with 1 (1-ounce) (28 grams) slice sharp Cheddar cheese.
Pro Tip
Pouring the béchamel sauce onto the griddle allows you to achieve that golden, cheesy top you expect from a croque monsieur.
