Okay, so it wasn’t toasted so much as fried, but yum. Two slices of wholegrain bread, a wedge of pecorino (Pecorino Giglio Sardo from my local Italian deli, Gallo Nero in Stoke Newington), two rashers of smoked pancetta (also from Gallo Nero). Served with green tomato chutney. Pecorino is ideal because it has the right combination of density with a bit of elasticity that allows fine slicing, without rubberiness - a good mature gruyere would do, or maybe manchego – and strength of flavour without being overwhelming, and even a little hint of sweetness. I think a properly mature cheddar would be too crumbly and too astringently powerful, although I’m not going to say it might not work if that’s what you like.
It is essential to slice the cheese finely, as it’s not going to be exposed to direct heat, but you do want it melty, so just thinly cover one slice of the bread. Meanwhile, briefly fry the rashers of pancetta, then lay them over the pecorino and cover with the other slice, then fry the sandwich in the pan with the pancetta fat and maybe a little olive oil (depending on how much fat the pancetta’s left behind). When you first put the sandwich in the pan you’ll want to flip it almost immediately, otherwise all the pork fat will be soaked up by just one slice of the bread, and you want it evenly distributed. Then, after the initial flip, just a minute or two on each side to get the bread golden brown and the cheese just melting. I happen to have a heavy Le Creuset casserole lid that fits neatly within my frying pan and I used that both to cover the sandwich, to retain heat and encourage the cheese to melt, and to apply some downward pressure to it, which I guess roughly duplicated at least some of the effect of those deli style sandwich toasters. That probably wasn’t essential, but I’m sure it helped.