Slice off the tip the garlic bulb to expose the cloves, drizzle with olive oil and wrap in foil. Pop in the oven for 45-60minutes in 356°F or till golden-brown and soft as butter.
Squeeze the garlic out & blend with a pinch of salt to fully smooth out it. Add into a large saucepan with butter and stir to blend. Stir in flour to make a paste, then pour in your milk, whisking as you go to prevent lumps. Once thickened, add mustard then seasoning, then stir into your cheese (remember to spare half parmesan).
Meanwhile, cook pasta in salted boiling water until just cooked. You need it still somewhat difficult. Reserve a cup of starchy-pasta water (see notes) then drain and combine with cheese-sauce.
Add everything to a baking dish, top with parmesan then breadcrumbs & complete with a pinch of salt & oil spray and pepper. Pop in the oven in 390°F for 20minutes or until golden on top. Sprinkle fresh-chives on the top and then serve.