In a cup, mix together cold water and apple cider vinegar, and place them into the freezer until needed.
Sift together the gluten free flour, coconut flour, xanthan gum and salt.
Add the ice cold water-vinegar mixture, 2 tbsp at a time, and mix the pie dough with a fork until it comes together. You will need 10 – 12 tbsp of water.
The dough at the end should be slightly more damp/wet than typical pie crust because gluten free flour tends to absorb more moisture during baking, so we want to counteract that.
Whisk together the eggs, milk and pepper until homogeneous.
Fry the broccoli florets and garlic in the olive oil until the florets are slightly softened.
Season to taste.
Fry the cubed pancetta until crispy, then remove from the pan and add the olive oil to the fat released by the pancetta.
Add the finely sliced onions and season with a pinch of salt and pepper. Cook the onions until caramelised.
If using fresh spinach: wilt the spinach by cooking it with 1 tbsp of olive oil in a covered pan. Season to taste.
If using frozen spinach: cook the thawed (and, if necessary, drained) spinach for a minute or two in 1 tbsp of olive oil. Season to taste.
Cook the finely sliced onions, seasoned with a pinch of salt and pepper, in 1 tbsp olive oil until caramelised.
Pre-heat the oven to 390 ºF (200 ºC), and get a 12-hole muffin/cupcake tin ready to have on hand.
Roll out the chilled pie dough about 2 mm thick, and cut out circles of with a round cookie cutter at least 3.5 inch (9 cm) in diameter. Re-roll the cut-offs and re-use them. In the end, you should have 12 pie dough circles.
Place each pie dough circle snugly into each muffin tin hole, and trim away any excess pie dough.
Place a cupcake/muffin liner into each pie dough cup, and fill it with rice.
Blind bake at 390 ºF (200 ºC) for 8 minutes, then reduce the oven temperature to 355 ºF (180 ºC), remove the rice and cupcake/muffin liners, and bake uncovered for a further 5 minutes.
Divide the fillings among the 12 pastry cups (makes 4 broccoli, 4 pancetta & caramelised onions, 4 spinach mini quiches). Sprinkle some grated cheddar cheese on top and pour over the "custard" until each cup is just before overflowing with it.
Bake in the pre-heated oven ay 355 ºF (180 ºC) for 26 to 28 minutes, until the filling is lightly puffed up, no longer wobbly and golden brown on top.
Remove from the muffin tin and allow to cool on a wire cooling rack.
The mini quiches keep well in an air-tight container in the fridge for 3 to 4 days. You can easily reheat them in the oven (at 355 ºF/180 ºC for about 10 to 15 minutes) or in the microwave (on the high setting for 1 to 2 minutes).