In a stand mixer fitted with the dough hook, process flour, sugar, salt and yeast until combined, about 2 minutes. You may need to scrape the sides and bottom of the bowel to incorporate all ingredients.
With machine running, slowly add water and process until dough is just combined and no dry flour remains. Run on medium for about 3-5 minutes to knead fully. Add extra flour ½ teaspoon at a time if necessary. The spelt will be a bit stickier but avoid adding too much extra flour or it will become heavy.
Let dough stand 10 minutes.
Remove dough from bowl and knead briefly on lightly oiled countertop until smooth, about 1 minute.
Shape dough into tight ball and place in large, lightly oiled bowl or covered container such as a rectangular tupperware. Cover tightly and refrigerate for at least 24 hours and up to 3 days.
Baking:
One hour before baking, adjust oven rack to second lowest position, set pizza stone on rack, and heat oven to 500 degrees. (You may want to keep the vent on and disconnect any nearby smoke alarms).
Remove dough from refrigerator and divide in half. Shape each half into smooth, tight ball. Place on lightly oiled surface, cover loosely and let stand for one hour.
Coat 1 ball of dough generously with flour and place on well-floured countertop. Using fingertips, gently flatten into 8-inch disk, leaving 1 inch of outer edge slightly thicker than center.
Using hands, gently stretch disk into 12-inch round, working along edges and giving disk quarter turns as you stretch. Transfer dough to well-floured peel and stretch into 13-inch round.
Toppings:
Slice the potatoes very thin, using the slicing blade of a food processor, Robot Coupe or a mandoline if desired.
Heat the olive oil in a pan and sauté the onions over medium heat until translucent, about 6 minutes.
Remove from the heat and toss with the potatoes and fresh herbs. Layer half the mixture over one of the pizza dough, add salt as desired and sprinkle half the cheese over the top. Be sure pizza peel has enough flour for pizza to slide around and is not sticky—check and double check as you go.
Slide flatbread carefully onto stone and bake until crust is well browned and cheese is bubbly and beginning to brown, about 6 minutes.
Remove flatbread and place on wire rack for 5 minutes before slicing and serving. Repeat to shape, top, and bake second flatbread.