To make the bechamel, get a medium saucepan out and heat the milk, whole spring onions, salt and pepper in a pot until bubbles form around the outside, and immediately at that point take it off the heat and pour into a large jug or bowl that's easy to pour from.
Measure your flours so that they're ready to go and clean saucepan and pop it back on the heat and melt the butter into it, being careful not to burn it.
Turn the heat off for a minute, while you whisk the flours into the butter until you get a thick, pastry looking ball
Now switch the heat back on medium and get a stick blender out. Let the 'roux' cook off for a minute until you can smell a delicious, rich pastry smell then add a tiny splash of the hot milk mixture and take your stick blender and combine. The first couple of milk additions will have it looking like a hot mess and that's TOTALLY NORMAL. Exhibit A!
Keep adding about 1/4-1/3 cup of milk each time and mix for about 4 rounds, then you can increase to 1/2 cup for the last 4 rounds of milk adding and handheld mixer smoothing. Blitz with the stick blender on high until it's a nice, smooth consistency and all the spring onions are blitzed through it too (my secret flavour weapon!)
Now, add your 2 cups of grated cheddar to the bechamel and stir through until melted. This will serve to thicken and add flavour. If it's runny at this point after those 2 cups, add your lucky 3rd emergency cup, but you shouldn't need it if you've used all the same flours/measurements as above.
You are ready to assemble you clever thing. It's downhill from here.