Cook the buckwheat (in salted water) according to packet instructions. Place in a medium size (about 6 cm deep) baking dish and set aside.
Preheat the oven to 450 F/ 230 C/ gas mark 8.
In a saucepan gently melt the butter, add the flour and whisk until the mixture is smooth. Add some of the milk, increase the heat and continue cooking until the mixture has thickened whisking all the time. Pour in the rest of the milk and cook until the sauce starts to bubble up whisking frequently.
Remove from the heat, add the cream cheese, mustard, onion granules and 2/3 of the grated cheese and stir. Adjust the seasoning as necessary.
Pour the sauce over the buckwheat, stir thoroughly ensuring the sauce is distributed evenly, scatter the remaining cheese over the top and bake for 12-15 minutes or until the cheese is golden brown. Remove from the oven and serve immediately.
Buckwheat: Boil the buckwheat in salted water according to packet instructions and either make the recipe or cool and then refrigerate the groats to use later. When you are ready to make the recipe just rinse the cooked buckwheat with a bit of hot water (in a strainer) and give it a good stir.
Best served immediately. The dish won't be as creamy after 30 minutes.
Not suitable for freezing.
Preparation Time includes the time needed to cook the buckwheat.