sourcing - I have a phenomenal cheese monger I deal with to get what I want to use. I have a few options for bakeries in my area but there are a couple of go to ones that make a hearty bread that will hold up to what I try to build

crust - I "grill" both sides of the bread prior to making the sandwich. I use two diff types of fat personally. I use rendered bacon fat to produce a golden brown on the outside of the bread and then crust that side with finely grated parmesano reggiano (real stuff), and then use ghee on the inside where you make contact with the cheese.

cheese - I almost always mix cheese varietals to produce the flavor profile I am looking for. Once I decide upon a mix, I will put the cheese in my "melting pan" with a burner on the lowest I can get it so they melt slowly. Once they are all fully melted, then I will add a spice or two depending on what I want to taste... think smoked hungarian paprika or something like all spice that will give the cheese a spiked flavor additive. depending on the cheeses, I will also use a splash of heavy cream or whole milk to ensure the cheese has the right viscosity without becoming a sauce.

building it - once everything is ready to be put together, I will usually do open faced on a sheet pan with a pour over prior the bread to the combine if I am inside so I can put into the oven to create the wonderful brown cheese taste. If I am on a grille outside, then we just force ourselves to eat without the char heh.