Heat the butter in a saucepan over a high heat. When the butter has melted, add the flour and with a whisk combine to a paste.
Reduce the heat to medium and slowly pour the milk into the pan, whisking constantly until all the milk is combined and the sauce is smooth. Stir through the grated cheese until melted, add the onion granules and crushed garlic. Remove from the heat and set aside.
Chop the spinach into small pieces and add to the cheese sauce and stir. Pour the cheese and spinach into a baking dish.
Bake in the oven for 30 mins and serve straight from the dish.