Put the carrot (no need to peel it), zucchini and celery in a food processor and blitz until finely minced. You want to hide the veg as much as possible (because who puts carrot, zucchini, and celery in their cheeseburger?).

Add a splash of veg oil to a large pan on high heat. Throw in the onions and fry for about 10 minutes until they start to get nice and brown. Add the minced veg from Step 1 and season with salt and freshly cracked black pepper. Keep frying another 3 or 4 minutes until the veg have sweated down and started to brown. Add the garlic and fry a final minute. Remove the mixture to a bowl and let it cool to room temperature.

Add the mince, panko, and egg to the bowl of veg. Throw in a good pinch of salt and several grinds of black pepper. Use your hands to gently fold the ingredients together until they're just combined. Don't knead too vigorously or for too long, or you'll end up with a tough, chewy loaf.

Lay the bread slices in the bottom of a high-sided baking dish. They're there to soak up any excess oil. Spoon half the meat mixture on top and spread it out evenly. Spread the cheese in a layer over that, then finish with another layer of the remaining meat mixture. Bake in an oven preheated to 375 F for 1 hour.

Pull the loaf out of the oven and glaze the top with the barbecue sauce. Put it back in the oven on high broil for 15 minutes. Use a thermometer to check for doneness (you want 160 F internal temperature). If it's under, cover the dish loosely with foil and put it back in the oven at 400 F until cooked. Let the loaf rest 10 to 15 minutes before digging in.