Prepare dough: In a medium bowl, mix the water, salt and yeast. Stir in the flour until flour is completely incorporated. Cover the bowl and let the dough sit in a warm place for 1-2 hours until doubled. (Or leave on counter for up to 8 hours.)
When ready to bake, preheat oven to 475°F and cover a baking sheet with parchment paper.
Prepare filling: In a medium bowl, mix the tomatoes, Italian seasoning, garlic powder, onion powder, salt, and pepper.
Turn dough out onto a lightly floured countertop. Roll out dough into a large rectangle, about the size of the baking sheet.
Spread the sauce evenly over the dough.
Sprinkle shredded cheese on top of the sauce, then top with the pepperoni.
Folding lengthwise, take the top 1/3 of the dough (long end) and bring it over to cover the center of the dough. Repeat with the other side, almost like you're folding a letter.
Place the stromboli, seam side down, on the prepared baking sheet.
Using a sharp knife, cut slits in top of dough, about every 1-2 inches. Brush the dough with the beaten egg and then sprinkle with 1 teaspoon of Italian seasoning.
Bake for 10 minutes. Then lower temperature to 375°F and bake for another 10-15 minutes until the dough is set and golden.
Let the stromboli sit for at least 10 minutes before slicing.