Steps:

• Preheat oven to 375 degrees F (190 degrees C).
• Slice the pork almost in half, but do not cut all the way through. Butterfly the sides until the pork is open enough to stuff.
• Mix torn bread, melted butter, poultry seasoning, onion, and celery in a bowl until bread is evenly coated with butter and seasoning; season with salt and pepper. Stuff the pork tenderloin with the bread mixture. Wrap the pork around the filling and secure closed with toothpicks. Wrap bacon slices around the stuffed tenderloin. Season top of tenderloin with black pepper.
• Cook in the preheated oven until pork is cooked through, about 45 minutes. An instant-read thermometer inserted into the center of the stuffing should read at least 145 degrees F (63 degrees C).

Steps:

• To prepare the filling, rub five-spice powder evenly over pork. Heat a grill pan over medium-high heat. Coat the pan with cooking spray. Add pork to pan; cook 18 minutes or until a thermometer registers 155°, turning pork occasionally. Remove pork from pan, and let stand 15 minutes.
• Cut pork crosswise into thin slices; cut slices into thin strips. Place pork in a medium bowl. Add onions and next 7 ingredients (through 1/4 teaspoon salt); stir well to combine. Cover and refrigerate.
• To prepare dough, combine 1 cup warm water, sugar, and yeast in a large bowl; let stand 5 minutes.
• Lightly spoon flour into dry measuring cups; level with a knife. Add flour, oil, and 1/4 teaspoon salt to yeast mixture; stir until a soft dough forms. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 10 minutes). Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.)
• Punch dough down; let rest 5 minutes. Turn dough out onto a clean surface; knead in baking powder. Let dough rest 5 minutes.
• Divide dough into 10 equal portions, forming each into a ball. Working with one dough ball at a time (cover remaining dough balls to keep from drying), roll ball into a 5-inch circle. Place 1/4 cup filling in center of dough circle. Bring up sides to cover filling and meet on top. Pinch and seal closed with a twist. Repeat procedure with remaining dough balls and filling.
• Arrange 5 buns seam side down, 1 inch apart, in each tier of a 2-tiered bamboo steamer. Stack tiers; cover with lid.
• Add water to a large skillet to a depth of 1 inch; bring to a boil over medium-high heat. Place steamer in pan; steam 15 minutes or until puffed and set. Cool 10 minutes before serving.