Heat 125ml of milk and 125ml of cream with 1 crushed garlic clove, 2 bay leaves, and 1 small chopped onion until nearly boiling. Infuse off the heat for 10 minutes, then strain.
Melt 20g of butter, stir in 20g of all-purpose flour, and cook until it smells toasty. Gradually whisk in the infused milk until thick. Season with 1 pinch of ground nutmeg.
Brush 4 slices of white sourdough with 20g of melted butter and toast one side under the broiler. Preheat the oven to 425°F / 220°C.
Spread the untoasted sides with 1 teaspoon of Dijon mustard and some béchamel. Top two slices with 4 thin slices of smoked ham and half of 100g of grated Gruyère; cover with the others, toasted side up.
Spread the remaining béchamel on top, sprinkle with the rest of the Gruyère, and bake for 10-15 minutes, until golden.
