1. Prepare the pasta: In a small saucepan, heat the milk, onion, garlic, and bay leaf until boiling point. Remove from heat, let infuse covered for 10 minutes, then set aside. Cook the macaroni al dente according to package instructions. This will take about 10 minutes. Drain in a colander, then rinse under the faucet and stir to prevent them from sticking together.
2. Prepare a roux (a roux is simply flour and fat cooked together, then used to thicken sauces.): Preheat the oven to 190C/fan 170C/gas 5 and butter a 25 x 18 cm oven-safe dish. Melt the butter in a medium saucepan. When foamy, add the flour and cook, stirring constantly, for 1 minute over low heat.
3. Finish the sauce: Gradually incorporate the warm infused milk into the roux until smooth. Let simmer for 3-4 minutes, stirring often, until the sauce has thickened and has a glaze-like consistency (run your finger through the sauce in a spoon – it should leave a trail). Remove the saucepan from the heat, then add the cheddar and mustard powder. Season, then stir until the cheese is melted.
4. Assemble and bake: Mix the cheese sauce and macaroni to coat well, then pour into the prepared dish. Sprinkle parmesan and breadcrumbs on top, then bake in the oven for 15 to 20 minutes until golden brown and bubbly. Serve hot, alone or with a green salad.