Mix the flour, cornmeal, sugar, baking powder, and 1/4 teaspoon salt. Combine oil, egg, and water. Stir liquid mixture into dry mixture until well blended, forming a ball.
On a lightly floured piece of parchment or waxed paper, roll or pat the dough into an 8-inch square. Combine the cilantro, chili powder, 1/4 teaspoon salt, and pepper. Rub cilantro mixture on surface of pork, coating evenly. Place tenderloin across center of dough, folding under narrow end of tenderloin to make an even thickness. With the aid of the parchment or waxed paper, roll dough around tenderloin. Seal edge and ends. Place seam side down on a lightly greased baking sheet.
Roast in a 425 degree F oven 20 minutes. Cover loosely with foil; roast 15 to 20 minutes more or until a meat thermometer inserted in center of meat registers 155 degrees F. Let stand 5 minutes. Serve with salsa. If desired, garnish with lemon leaves. Makes 4 servings.