Grate the 3 cheeses and combine them all in a bowl.
Arrange the slices of bacon on a sheet tray, preferably with a wire rack. Place in a cool oven and then turn the oven on to 375 F degrees. Cook for 20 minutes and then flip the bacon. Cook for another 20 minutes and the bacon should be beginning to crisp up. Remove any pieces that are perfectly cooked and then continue cooking the rest a few minutes at a time until they are all perfectly cooked. Remember that they will firm up as they cool and they carry over cook so pull them just before you think they are perfectly crispy and they will be perfect.
Slice the heirloom tomato and then set on a paper towel to dry the excess moisture.
Assemble the Sandwich
On the old food truck and now at home, I like to make my sandwiches in a small quarter sheet tray to contain any cheese that spills out while assembling. Using a 1/3 cup measuring cup, pack the measuring cup with the cheese blend and then evenly spread on the bottom piece of bread. Next, take 3-4 slices of the bacon and just tear them into small bite sized pieces, place half on top of the cheese. Then hit 2-3 pieces of tomato (depending on size) with some salt and add on top of the bacon. Then add the rest of the torn bacon on top of the tomato, then one more 1/3 cup of cheese. Close the sandwich and gently compress it. Any cheese that fell out of the sandwich you can gather and place back in the sandwich.
Cooking the Sandwich
Preheat the cast iron pan on medium heat. Once hot add the sandwich to the pan dry, without any fat. Lightly toast one side of the bread and then flip and toast the other side of the bread. While that side is toasted, smear the thinnest layer of mayo from edge to edge on the toasted side. Use the edge of a spreader to spread it thinly and evenly but also to scrape off any excess. Once the other side is lightly toasted, flip the sandwich and repeat. Once both sides are nicely toasted and browned, lower the heat and keep the sandwich moving like a hot potato. Flip every 30 seconds or so to avoid developing too much more color on the bread but giving the cheese time to melt. Once the cheese is melted, let the sandwich rest for 1 minute, and then it's ready to serve. Make sure to have some hot soup ready when the sandwich is done.
Slice the sandwich, serve with some roasted tomato soup with a garnish of basil and use the soup as a dip for the sandwich.