Pre-heat your oven to 180C
Place the cubes of chopped butternut squash into a microwavable bowl and add the water. Cover the bowl with cling film and pierce a hole in the top with a knife
Microwave the squash for 15 minutes on high, or until soft. You can check it via the hole in the cling film
When it is done, drain off the water and use a fork to roughly mash the squash. If you want a smoother puree, use a blender instead
Meanwhile, set some water to boil and add the pasta. Boil according to the packet instructions, usually 10 minutes, then drain and add to your baking dish
To make the cheese sauce, melt the butter in a saucepan on a low heat. Add the garlic into the melted butter and let it cook for a couple of minutes
Add the plain flour and whisk it into the butter, let it cook for 1 minute
Then start adding the milk slowly, bit by bit, with each addition whisk the milk well into the flour to avoid a lumpy sauce
When all the milk is added, add the cream and stir it in
Turn up the sauce to a medium heat, stir it constantly with a silicone spatula making sure to scrap along the bottom and sides of the pan as you stir (this is stop the sauce from catching) keep doing this until the sauce thickens, around 3-5 minutes
Turn off the heat and add the cheese, stir until it has melted into the sauce. Add the sage and salt, if using
Add the mashed up squash to the cheese sauce and mix it in
Pour the sauce over the pasta, stir the mixture gently so that the sauce evenly coats the pasta
Top with the grated cheese and breadcrumbs
Bake in the oven for 25-30 minutes, or until golden on top and bubbling