Preheat the oven to 180°C. Grease a 9-inch cast iron skillet (or pie pan) with nonstick cooking spray.
In a large bowl <g class="gr_ gr_85 gr-alert gr_gramm gr_inline_cards gr_run_anim Punctuation only-ins replaceWithoutSep" id="85" data-gr-id="85">mash the potato and slowly incorporate a beaten egg, rice flour and a generous pinch of salt. Mix until you have a smooth, quite solid mixture, adding a bit more flour if necessary.
Spread the sweet potato mixture over the base and edges of the skillet.
Bake in the oven for about 15 min, until the <g class="gr_ gr_83 gr-alert gr_gramm gr_inline_cards gr_run_anim Punctuation only-ins replaceWithoutSep" id="83" data-gr-id="83">base is quite solid and crunchy, but not too cooked.
In the meantime, prepare the filling. In a bowl, with the help of a fork beat the eggs lightly with the milk, ½ tsp of sea salt, and ½ teaspoon of pepper (or to taste). Fold in the spinach leaves and chopped bell pepper and lightly combine all the ingredients.
Remove the crust from the oven, and pour the filling over the base, distributing evenly the spinach and peppers.
Transfer the quiche back in the oven and bake for 20 minutes, or until the filling is no longer creamy and runny.
Remove from the oven and allow the quiche to cool for about 10 minutes, then slice into wedges and serve.