In a large mixing bowl, whisk together flour, salt and yeast. Add water and mix until a shaggy mixture forms. Cover bowl with plastic wrap and set aside for 12-18 hours. Overnight works great.
Place a cast iron dutch oven into oven and heat to 450 degrees F. Heat pot for 30 minutes. Meanwhile, pour dough onto heavily floured surface and shape into a ball. Cover with plastic wrap and let set while the pot is heating.
Remove hot pot from the oven and drop in the dough. (Don't worry if the shape is ugly at this point. It will even out in the baking process.) Cover and return to oven for 30 minutes. After 30 minutes, remove the lid and bake an additional 15 minutes. Remove bread from oven and place on a cooling rack to cool.
Although spelt is actually a type of wheat flour, most people with wheat allergies can digest spelt with no problem. Obviously, you should not eat spelt if it is a problem for you.