Preheat the grill. Whisk together the mustard, honey, horseradish and mint in a bowl and season with salt and pepper, to taste. Let sit at room temperature for at least 15 minutes to allow the flavors to meld.
Thread the tomatoes onto skewers and put them into a large bowl. Add the asparagus and drizzle with a little oil. Season with salt and pepper, to taste. Put the tomatoes on the grill and cook until charred on both sides. Grill the asparagus for about 2 minutes per side. Put the vegetables on a platter and drizzle them with the mustard sauce. Serve hot or at room temperature.