Heat oven to 400° F. Roll the pizza dough into a 14-inch circle and cut into 8 triangles.
Stack a slice of ham and cheese and some onion on the bottom of each triangle and roll the dough up around the filling. (Some will stick out the ends.)
Transfer the rolls to a parchment-lined baking sheet. Bake until golden and crisp, about 20 minutes.
Combine the mustard and oil in a medium bowl and toss with the romaine. Serve with the turnovers.