At least 2 to 3 hours before serving
Preheat the water bath to 141°F (60.6°C).
Place the bratwurst into the sous vide bag and add the beer. Seal the bag then place in the water bath and cook for 2 to 3 hours.
At least 12 hours before serving
Combine the mustard seeds, malt vinegar, and Guinness in a bowl. Cover with plastic wrap and let sit for several hours or preferably overnight.
The next day, add the soaked mustard seeds and their liquid to a blender or food processor. Add the remaining ingredients and process until it is the consistency you desire. The mustard can be refrigerated for several weeks.
40 minutes before serving
Peel the onions and then cut into slices about 1/2" to 3/4" (13mm to 19mm) thick, trying to keep the slices together. You can also thread the onion slices onto a shish-kabob skewer.
Cut the sides off of the peppers, leaving them whole. Salt and pepper the onions and peppers and then drizzle with the canola oil.
Heat a grill to high heat. Add the onions and peppers and cook until they just begin to brown and are cooked through. Remove them from the heat. Once they have cooled enough to handle slice the peppers into 1/2" (13mm) strips and cut the onions in half.
Remove the bratwurst from their bag and pat them dry. Sear the brats on the grill over high heat, about 1 or 2 minutes per side.
Brush the inside of the buns with oil and then grill briefly until they just start to brown. Remove from the heat then smear some Guinness mustard on the buns, add a bratwurst, and top with the peppers and onions then serve.
Because they use modernist ingredients, the amounts for this component are given in metric by weight. Learn more about how to measure modernist ingredients in this article.
Adding 0.3 grams of xanthan gum to the Guinness Mustard before blending it helps it hold together better and keep its shape when plating.