Slice the tip off the garlic bulb to expose the cloves, drizzle with olive oil and wrap in foil. Pop in the oven for 45-60mins at 356F/180C or until golden brown and soft as butter.
Squeeze out the garlic & mash with a pinch of salt to completely smooth it out. Add to a large saucepan over medium heat with 3 tbsp butter and stir to combine. Stir in 3 heaped tbsp flour to form a paste, then very gradually pour in 1litre/4cups milk, whisking as you go to avoid lumps. Important to make sure the sauce thickens before adding the cheese, don't use the cheese to thicken the sauce. If it's watery by the time you add all the milk just rapid simmer until it can coat the back of a spoon, then stir through cheeses (step 4).
Meanwhile, cook 14oz/400g pasta in salted boiling water until only JUST cooked. You want it still a bit hard. Reserve a cup of starchy pasta water then drain and combine with cheese sauce. Pre heat oven to 200C/390F.
Once thickened, add 1 tbsp dijon mustard and seasoning (to taste), then stir in 2cups/200g cheddar, 2cups/200g mozzarella and 1/2cup/40g parmesan (other half is to top). It may seem overly stringy at this point, but the small amount of pasta water attached to pasta will smooth it out.
Add everything to a baking dish, top with 1/2cup/40g parmesan then 1/2cup/30g breadcrumbs. Finish with a pinch of salt & pepper and oil spray. Pop in the oven for 20-25mins or until golden on top. Sprinkle fresh chives over the top and serve.