1. Position two heavy-duty aluminum-foil wrapped firebricks about 4 inches apart on one area of a well-oiled charcoal or gas grill, and preheat all areas to high.
2. Combine the beer bath ingredients in a baking dish or disposable aluminum pan, preferably a 13 1/2 X 9 5/8 X 2 3/4 -inch lasagna pan. Place the uncovered dish on the bricks, close the lid and preheat until hot, about 45 minutes.
3. Add the bratwurst to the bath. It’s okay if the bratwurst touch and are not completely submerged in the bath. Close the lid and cook for 20 minutes, flipping the bratwurst once halfway through the cooking.
4. Meanwhile, pour a film of canola oil into the baking dish or disposable pan.
5. Remove the bratwurst from the bath, roll in the oil lightly to coat, and place evenly across the grate with about an inch between each. Turn to brown and caramelize deeply on all sides, about 2 minutes per side.
6. The brats can be served at this point, or held in the beer bath over very low with the lid closed for up to 1 hour. Occasionally test the heat of the bath. The bath should remain warm, 140 degrees F, but should not boil.
Serve the brats on soft rolls, topped with onions and/or sauerkraut, and mustard.
For Grilled Sweet Onions:
1. Cut the onions through the root end and lay, cut side down, on the work surface. Cut across each half, following the natural lines, to make perfectly even 1/8-inch slices. You will need 6 cups.
2. Place a cast-iron griddle on a well-oiled charcoal or gas grill. Preheat all areas to high.
3. Toss the onions with the 2 tablespoons of the olive oil, thyme, sugar, salt, and pepper.
4. Pour the remaining 1 tablespoon of oil on the griddle and let heat for about 1 minute. Spread the onions on the griddle and decrease the heat to medium. Close the lid and cook the onions, without stirring, for 10 minutes.
5. Move the onions to one side of the griddle. Melt the butter on the cleared space, add the garlic, and stir to coat in the butter. Cook until fragrant, about 1 minute. Stir the onions and garlic together and cook with the lid open until the onions are completely tender, about 3 minutes. Pour the vinegar right onto the griddle and mix into the onions.
6. Serve directly from the grill or transfer to a bowl.