How to Make It
Light a grill or preheat a grill pan. Working over a medium bowl, grate the tomato halves on the large holes of a box grater; discard the tomato skins. Stir in the marinara and Sriracha and season the dip with salt.
In a large bowl, toss the potatoes with the olive oil and season with salt. Grill over moderately low heat, turning halfway, until the potatoes are golden and cooked through, about 10 minutes. Transfer to a plate and season with salt. Serve the hot potatoes with the dip.