In a non-reactive casserole just large enough to fit all the squab in one layer, combine the honey, vinegar, olive oil, thyme leaves, and onion and mix well. Add the squab and turn to coat, making sure that as much of each bird is in contact with the marinade as possible. Cover and refrigerate for at least 4 hours (up to 8 hours), running the squab occasionally.
Preheat the grill or broiler. Place the bricks on the grill and heat until very hot.
Remove the squab from the marinade and pat dry with paper towels. Season with salt and pepper. Place the squab breast down on the hottest part of the grill, carefully place a hot brick on top of each and cook for 6 minutes on each side. Then, using tongs, remove the brick, carefully turn the squab, replace the brick, and grill the other side for 4 to 6 minutes or until medium rare, about 130 degrees F on an instant-read thermometer.
To assemble the dish, place some of the farrotto in the center of each of four warmed dinner plates. Place a squab atop each mound of farrotto and top with a tablespoon of porcini mustard.