Grilled Beer Brats with Onions, Bell Peppers & Roasted Green Chile

In a 4-quart (3.8-l) stockpot over medium-high heat, combine the yellow onion, salt, bratwursts, beer, and just enough water to cover the brats and and bring to a low boil. Reduce the heat to a simmer and simmer uncovered, turning the brats occasionally, until firm, 30–40 minutes.
While the brats are simmering, in a seperate pan over medium-high heat, melt the butter. Add the white onion and bell peppers and cook, stirring occasionally, until tender, 5–8 minutes. Stir in the chiles and cook until the chiles are well mixed with the onion and peppers and heated through. Remove from the heat and keep warm.
Prepare a fire in a charcoal or gas grill for direct grilling over medium-high heat.
Remove the sausages from the beer broth and place directly over the fire. Grill, turning as needed, until browned on all sides, 2–4 minutes on each side. Return them to the pot with the beer broth to keep warm until ready to serve.
To serve, tuck a brat into each warm, crusty bolillo, top with the onion-chile mixture, and swab with a little mustard. Serve at once.