Bacon, caramelised onion, thin apple and white cheddar between multigrain bread, with mustard on one side and mayonnaise on the other.

One sandwich, twenty five minutes, and every component gets cooked in the same skillet.

Mayonnaise inside, butter outside

Which is the reverse of what people often do, and it is right.

Mayonnaise on the inside face acts as a moisture barrier between the bread and the fillings, and it adds richness and a little acidity where you taste it.

Butter goes on the outside, where it browns. Some cooks use mayonnaise on the outside for browning too, and it works, but butter gives a better flavour and this recipe has enough richness already.

Room temperature butter spreads without tearing the bread. Cold butter drags.

Brown sugar with the onions

A teaspoon, added with the onions and a tablespoon of butter into the bacon pan.

Onions caramelise on their own given twenty or thirty minutes. Five minutes is not enough, so the sugar accelerates it, giving colour and sweetness in the time available.

It is a shortcut rather than the real thing, and in a sandwich with this much else going on it is a reasonable trade.

Cooking them in the bacon fat is the other half. They pick up smokiness that plain butter would not give.

Same skillet throughout

Bacon first, then onions in the rendered fat, then the sandwich itself.

Each stage builds on the last. The bacon leaves fat and browned bits, the onions pick those up, and the sandwich toasts in what remains.

Blot the grease from the bacon and onions if you want, which the recipe offers. This is already a rich sandwich at 636 calories and 52 grams of fat.

Cheese on both sides of the fillings

Half the cheddar under the onions, apple and bacon, half over them.

Melted cheese is the glue. Fillings sandwiched between two cheese layers are held in place; fillings sitting on top of a single layer slide out when you bite.

Three ounces grated across one sandwich is generous, and it needs to be for that to work.

Grate it rather than slicing. Grated cheese melts faster and more evenly, which matters when you are trying to melt it through before the bread burns.

Medium-low heat, two to four minutes a side

Lower than you might expect for a grilled cheese.

The problem with a loaded sandwich is that the bread browns long before the cheese in the middle melts. Medium-low slows the outside enough for the inside to catch up.

If your bread is golden and the cheese is still solid, the heat was too high. Turn it down and give it longer.

Thin apple slices

Half a Granny Smith, sliced thin.

Thin is what lets them soften slightly in the pan and sit flat in the sandwich. Thick slices stay crunchy and raw and they make the sandwich awkward to bite through.

Granny Smith is tart, which is doing real work against bacon, cheddar and caramelised onion. A sweet apple would tip an already rich sandwich.

Whole-grain mustard on the top slice

A tablespoon, spread on the bread that goes mustard-side down onto the cheese.

Mustard is the sharpest thing in the sandwich and it cuts through the fat. Whole-grain gives texture and a milder heat than smooth Dijon.

Putting it on the top slice facing in means it sits against the cheese rather than against the apple, where it would compete.

The sodium

Around 906mg for one sandwich, which is high for a single serving.

Bacon, cheddar, salted butter and mustard all contribute. Unsalted butter is the easiest reduction, and there is enough salt elsewhere that you would not notice.

Serving

Sliced in half, hot. This is a substantial sandwich and it does not need much alongside. A sharp pickle or a simple green salad if anything. Bread choice Hearty multigrain is specified and the density matters. A soft white loaf collapses under this many fillings and goes greasy from the butter.

Sourdough works well and adds its own tang. Rye is a good match for the mustard and bacon.

Scaling up

The recipe doubles or triples, though you will need to cook the sandwiches in batches unless you have a large griddle. Keep finished ones in a low oven on a rack rather than stacked, so the bottoms stay crisp.