Start with the sauce; in a saucepan, melt the butter over medium heat with the garlic and onions for 2 to 3 minutes.
Gradually stir in the flour to create a roux. Slowly whisk in the milk until simmering and the mixture is smooth.
Turn the heat to low and stir in the mustard, beer, and cheddar cheese a little at a time until fully melted, thick and bubbling. Keep warm until ready to drizzle over the sausage and bread.
Preheat the barbecue to medium or until the charcoal ashes turn white.
Place the sausages over indirect heat and cook for 8 minutes. Switch to a high heat for the last few minutes to help colour the sausages.
Once the sausages are cooked, warm the bread rolls and drizzle some mayonnaise or butter on the base. Add the salad leaves, the sausages and then your sauce.