Cook potatoes: Peel potatoes and cut into 3⁄4-inch pieces. Transfer to a medium saucepan along with 1 teaspoon salt; cover with 1-inch of cold water and bring to a boil. Cook until tender when pierced with a knife, about 8 minutes. Pour off the water and set pan over medium heat, shaking gently until potatoes are dry, about 1 minute. Remove from heat.
Prep ingredients: Meanwhile, trim ends from onion, peel, then slice into 1⁄2-inch rounds, keeping slices intact. Quarter bell pepper, removing stem, core, and seeds and press pepper to flatten slightly. Pick parsley leaves and coarsely chop; discard stems.
Grill vegetables: Preheat a grill or grill pan over high. Rub onion rounds and peppers all over with oil, add to grill and reduce heat to medium high. Cook, covered, until charred and tender, turning once or twice, about 8 minutes for onions and 10 minutes for peppers. Transfer to a plate. Reserve grill for step 5.
Make dressing: Meanwhile, in a large bowl whisk vinegar, mustard, and 2 tablespoons oil until combined. Add potatoes, season to taste with salt and pepper, and toss gently to combine.
Grill sausages: When onion and peppers are done, add sausages to grill or grill pan and cook over medium heat, turning until sizzling and lightly charred all over, 8–10 minutes. Transfer to a plate.
Finish and serve: Cut peppers into strips, and onion into quarters, separating the rings. Add onions, peppers, and chopped parsley to dressing and gently toss. Serve alongside sausages. Enjoy!