Load the wood tray with one small handful of wood chips and preheat the smoker to 225°F.
Place 6 bratwursts on the top shelf of the smoker. Smoke for 1 to 1 1/2 hours or until internal temperature reaches 160°F.
Heat oil in a large skillet. Saute onions and green peppers over medium-low heat, stirring frequently, for 10 minutes, or until onions are golden brown.
Add the garlic and celery seeds, and continue cooking for 5 minutes. Stir in the Dijon mustard, and cook 3 more minutes to blend flavors. Serve immediately, or refrigerate to serve at a later time.