Put the flour into a bowl; add salt; then slowly add 1 1/2 cups water, whisking to eliminate lumps. Cover with a towel, and let sit while oven heats, or up to 12 hours. The batter should be about the consistency of thin pancake batter.
When ready to bake, heat the oven to 450°F Put the oil in a 12-inch rimmed pizza pan or skillet (along with the onion and rosemary if you’re using them) and put in the heated oven. Wait a couple of minutes for the oil to get hot, but not smoking; the oil is ready when you just start to smell it.
Carefully remove the pan (give the onions a stir), pour in the batter, and return the skillet to the oven. Bake 30 to 40 minutes, or until the flatbread is well browned, firm, and crisp around the edges. (It will release easily from the pan when it’s done.) Let it rest for a couple minutes before cutting it into wedges or squares.
To use for pizza, top baked flatbread with a thin layer of tomato sauce, a sprinkling or crumble of cheese, and thinly sliced vegetables, cooked meat, olives, onions, etc. Turn on the broiler and put the pan under the heat until the ingredients are hot and bubbly. Let rest as above, then cut and serve.