To prepare the filling, heat 1 1/2 teaspoons canola oil in a large Dutch oven over medium-high heat. Add red onion, green onions and salt to pan; saute 12 minutes or until tender and browned, stirring often. Add vinegar, scraping pan to loosen browned bits. Add soy sauce, honey, five-spice powder and pepper; cook 2 minutes or until liquid evaporates. Cool to room temperature. Transfer to a bowl; chill 30 minutes.
To prepare dough, dissolve sugar and yeast in 3/4 cup plus 2 tablespoons warm water in a medium bowl; let stand 5 minutes. Stir in sesame oil. Add 2 1/2 cups flour to yeast mixture; stir until a soft dough forms. Turn dough out onto a floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoons at a time, to prevent dough from sticking to hands (dough will feel sticky). Lightly spray surface of dough with cooking spray; cover and let rest for 10 minutes.
Divide dough into 8 equal portions, shaping each into a ball. Cover dough balls loosely with plastic wrap, and let rest 5 minutes. Working with one portion at a time (cover remaining portions to prevent drying), flatten dough with hand into a 3-inch circle. Place about 2 1/2 tablespoons onion mixture in center of circle. Bring up sides to cover filling; twist top of dough, pinch to seal (bun will be very full). Carefully place bun, seam side down, on baking sheet coated with cooking spray. Repeat procedure with remaining dough balls and onion mixture.
Cover buns with plastic wrap; refrigerate, and let rise 2 hours or until slightly puffed (buns will not double in size).
Heat 1 teaspoon canola oil in a large nonstick skillet over medium-high heat. Arrange 4 buns in a single layer, evenly spaced in pan; cook 2 minutes or until bottoms are crisp and golden brown. Pour 1 cup water into pan, and cover; steam 8 minutes or just until water evaporates. Transfer buns to a plate; cover and keep warm. Wipe pan with paper towels. Repeat with remaining 1 teaspoon canola oil, remaining 4 buns and remaining 1 cup water. Serve warm.