Preheat oven to 200 degrees. Combine pork, cabbage, green onion, red bell pepper, egg, ketchup, mustard, soy sauce, brown sugar, and salt & pepper in a mixing bowl. Set aside.
To form the dumplings, remove 1 wonton wrapper from the package, covering the others with a damp cloth so they won't dry out. Brush 2 of the edges of the wrapper lightly with water. Place ½ rounded teaspoons of the pork mixture in the center of the wrapper (Yes, the pork mixture is uncooked. It will cook completely through the thin wrapper, I promise).
Fold over, seal edges, and shape as desired. Repeat the procedure until all of the filling is gone.
Heat a 12-inch saute pan over medium heat. Brush with vegetable oil once hot. Add 8 to 10 potstickers at a time to the pan and cook for 2 minutes, without touching.
Once the 2 minutes are up, gently add ⅓ cup chicken stock to the pan.
Turn the heat down to low, cover, and cook for another 2 to 3 minutes. Remove wontons to a heatproof platter and place in the warm oven.
Clean the pan in between batches by pouring in water and allowing the pan to deglaze. Repeat until all the wontons are cooked. Serve immediately with Yoshida's sauce for dipping.