Wash spinach thoroughly and place in a large, dry pan.
Heat, with the lid on, for a few minutes, until the spinach starts to wilt. Do not overcook, the spinach should still have some bite in it and be a vibrant green colour.
Transfer spinach to a colander to drain while you make the cheese sauce.
To make the sauce, heat the butter in a medium sized saucepan over a medium heat until it starts bubbling.
Stir in the flour and cook, stirring constantly, for one minute.
Remove from the heat and whisk in the milk. Return to heat and stir until thickened; this will only take a few minutes.
Remove from heat and stir in 85 grams of grated cheese and the mustard.
Divide your spinach into six portions and arrange in a large casserole dish.
Pour the cheese sauce evenly over the top.
Sprinkle with the remaining 35 grams grated mature cheddar, and sprinkle with paprika.
Bake for 20 minutes until golden and bubbly. Serve immediately.