Heat the oil in a large, deep saute pan or Dutch oven set over medium-high heat. Add the bacon and cook until browned and crispy, 3 to 5 minutes. Add the onions, the green and red peppers and the garlic and cook for another 4 to 5 minutes, or until the peppers are wilted and lightly browned. Add the beer, chicken stock, thyme, bay leaf and rosemary and deglaze the pan. Prick holes in the brats with a fork or skewer to prevent them from bursting during cooking. Add the brats to the pan, reduce the heat and simmer gently for 15 to 17 minutes, turning the brats halfway through.
Preheat a grill or cast-iron griddle to medium-high heat.
Remove the brats from the pot; let the braising liquid continue to simmer and reduce. Pat the brats dry with paper towels, then score them diagonally, making 3 or 4 shallow incisions on either side (this will ensure plenty of crispy edges when grilled). Brush the grill lightly with oil and grill the brats for 2 to 3 minutes per side, or until well-marked.
When the braising liquid has reduced to the consistency of a sauce, remove it from the heat and stir in the mustard and butter. Taste and adjust the seasoning with salt and pepper.
To serve, spoon some of the braising sauce, along with the peppers and onions, onto a platter. Set the grilled brats on top and drizzle with the remaining sauce. Garnish with chopped parsley and serve.