Preheat oven to 425°F. Place a baking stone in the oven to heat. If you don't have a baking stone, lightly oil a rimmed baking sheet.

If using a baking stone, spread dough on piece of parchment so it will fit your baking stone; otherwise, spread out dough in rimmed baking sheet. Dough should be about 1/2 inch thick at the most. Cover with a clean, dampened, dish towel.

Heat oil in a large skillet over medium heat. Add onions and fry, stirring often, until beginning to brown, about 15 minutes. Add salt and continue cooking, stirring often, until onions are caramelized, about 5 minutes more. Remove from heat.

Using a fork, pierce dough every 2 inches.

Brush egg on bread dough and spread onions in a even layer on top. Sprinkle with freshly ground black pepper and poppy seeds.

Bake, on stone, if using, until dough is a golden brown.