How to Make It
Light a grill or preheat a grill pan. Brush the onion slices with oil and season with salt and pepper. Grill over moderate heat, turning once, until softened and lightly charred, about 3 minutes. Let cool, then coarsely chop.
Roast the bell peppers and poblano chiles on the grill or under a preheated broiler, turning, until charred all over. Let cool. Peel, seed and stem the peppers and chiles, then thinly slice them crosswise.
In a medium bowl, combine the peppers and chiles with the onion. Add the mustard, parsley, vinegar, sugar, thyme and the 1 1/2 tablespoons of olive oil. Season the relish with salt and pepper.
Heat a large cast-iron skillet. Prick the sausages with a knife and cook over moderate heat, turning occasionally, until browned and cooked through, about 10 minutes. Serve with the mustard relish.