Preheat the slow cooker and when it's warm, pour in the melted butter.
Sieve in the flour, stirring continuously to make a roux (see recipe notes), ensure any lumps of flour are smoothed out. Add the garlic granules, salt, pepper and mustard and mix through.
Slowly pour in the double cream stirring continuously, to form a thick sauce. Then add the milk gradually (again stirring through) to create a thinner creamy sauce.
Pour in the uncooked macaroni and make sure all of the pieces are submerged in the sauce. Cover and cook on low for 1.5 to 2 hours until the macaroni is cooked 'al dente' - when the pasta is soft but not bloated, and retaining its shape.
Add the grated cheddar and parmesan cheese and stir through. Replace the lid and cook for a further ten minutes. Then serve immediately.
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Notes
Roux
A roux is a combination of butter and flour used as a base for white sauces.
Sieve the flour whilst adding to ensure it's smooth.
Preheat the slow cooker before cooking, this will ensure a consistent temperature for making the roux and helps to avoid any lumps of flour forming.
Larger pasta shapes can be used (i.e. Penne, Rigatoni) however the cooking time will need to be increased.
Cheese
Avoid using pre-grated cheese, freshly grated has a sharper burst of flavour and will taste better in the sauce. Also, anti-caking agents can be included in pre-grated cheeses which will affect the consistency of the sauce giving a more gritty texture.