Make the gluten free pastry according to the recipe. The bring a big pan of water to the boil, then tip in the potatoes. Boil until just tender, then drain well, slice and allow to cool.
Heat oven to 200C/180C fan/gas 6 with a baking tray inside. On a large sheet of baking parchment dusted with a little gluten free flour, roll out the pastry into a rough circle about 30cm wide, until the thickness is about 10mm.
Brush the mustard over the gluten free pastry. Arrange the potatoes, spinach and most of the Gruyère in layers in the centre of the base, leaving a border of pastry that’s roughly 5cm all the way around.
Season and fold up the gluten free pastry edges to enclose the vegetables and cheese, pinching the edges to seal. Whisk together the lactose free cream and the egg with a little seasoning, then pour into the centre of the tart.
Sprinkle over the remaining cheese and slide the tart onto the preheated baking tray. Bake for 30 mins until the pastry is golden and crisp. Eat just warm or at room temperature.