How to Make It
Preheat oven to 425°F with rack in lowest position. Heat oil in a large skillet over medium-high. Add mushrooms, onion, salt, pepper, and oregano and cook, stirring, until vegetables are tender, 8 to 10 minutes. Add spinach and garlic and cook, stirring, until spinach has wilted, about 3 minutes. Transfer to a bowl and let cool slightly.
On a lightly floured piece of parchment, roll dough into a 10-by-15-inch rectangle. Spoon mushroom mixture length-wise down center of dough and top with tomatoes and cheese. Brush edges with egg and roll away from you into a log, pinching edges to seal. Place seam side down and transfer dough and parchment to a rimmed baking sheet. Brush with egg and cut 4 slits along the top. Bake on lowest rack until golden brown, 15 to 18 minutes. Let cool 15 minutes before slicing.