Preheat the oven to 200°C.
Heat 1 litre of milk with bay leaves in a pan until reaching scalding point.
Melt 40g butter in a separate pan and stir in 50g flour for 4 minutes.
Remove the bay leaves from the milk.
Gradually whisk the hot milk into the butter and flour mixture to form a smooth sauce.
Bring the sauce to a boil while stirring and simmer for 4 minutes.
Stir in 250g Gruyère, mustard, salt, and pepper after removing from heat.
Sauté shallots in butter, then stir in macaroni, tomatoes, and the cheese sauce.
Transfer to four bowls, top with Parmesan, and bake for 10 minutes.
Preheat the grill to high.
Grill the bowls until the cheese is bubbling and golden-brown.
Serve immediately in the hot cooking bowls.