Stab your potatoes a few times all around, then coat in a drizzle of olive oil and a pinch of salt. Pop in the oven at 356f/180c for 90mins, or until deep golden, crispy and knife tender.
Meanwhile, melt 3 tbsp butter in a pan over medium heat. Add 14oz/400g broccoli florets and fry for 5 mins until they JUST begin to pick up colour. Turn heat to low and continue frying for another 10mins or until they soften. Season with salt and pepper. (Alternatively you can roast the broccoli)
Turn heat back up to medium, stir in 2 tbsp flour and fry for a few mins. Gradually pour in 2cups/500ml milk, whisking as you go, until thickened and lump free.
Turn off heat and stir in 2cups/200g cheddar. Add 1 tbsp dijon mustard, 1 tbsp fresh chives and seasoning to taste. Place to one side.
When the potatoes are cool enough to handle, slice the very tops off. Gut out the centre, leaving a thin wall attached to thin skin (enough that will keep the filling from seeping out). Split broccoli cheese among potatoes and top with your remainder of cheese.
Pop back in the oven at 220c/430f for 10-15mins or until golden and bubbly on top. Allow to rest for a few mins so the filling thickens slightly then tuck in!