For the potatoes: Fill a large pot with cold water; add the potatoes and a pinch of salt. Bring to a boil, then reduce the heat to medium low and simmer until the potatoes are just fork-tender, about 15 minutes. Drain and let cool slightly, then cut into halves or quarters depending on the size
Preheat a grill to medium high.
Drizzle the potatoes and red pepper strips with olive oil and season with salt and pepper. Transfer to the grill and cook until charred and cooked through, 8 to 10 minutes (see Cook's Note). Chop the peppers once cool enough to handle.
For the steak: Season the steaks on all side by drizzling with olive oil, then sprinkling on salt and a few grinds of pepper. Place the thyme on the grill and immediately place the steaks over the top. Cook on one side for about 4 minutes, then flip, discarding the thyme, and grill for another 3 minutes. Remove the steaks from the grill and gently tent with aluminum foil for 10 minutes.
For the dressing: In a small bowl, whisk the mustard, vinegar, olive oil and sugar until the sugar granules dissipate into the dressing. Add the scallions and stir.
Cut the meat off the bone and chop the steak into bite-size pieces. Toss them in a large bowl along with the potatoes and peppers. Mix, then drizzle over the dressing and toss. Serve warm or at room temperature.
You can also roast the potatoes and peppers in the oven at 400 degrees F on a baking sheet drizzled with oil for 25 minutes.