For the marinade: In a medium bowl, whisk together the Dijon mustard, grainy mustard, the zest and juice from one of the lemons, white wine, fresh thyme, and the butter. Put the pork loin directly into the mixture, smoothing it on the meat like a paste. Refrigerate.
Preheat the oven to 350 degrees F. Heat a large skillet over high heat. Add 2 tablespoons of the olive oil. When the oil begins to smoke, remove the skillet from the direct heat. Remove the pork loin from the bowl, leaving behind any excess mustard mixture.
Reserve the mustard mixture. Season the meat on all sides with salt and gingerly place the pork loin in the hot oil. Put the skillet back on the heat and brown the pork, without moving it around, for 3 minutes. Use a pair of kitchen tongs (or two large spoons) to gently (but firmly) rotate the pork a 1/4 turn in the oil. Repeat this process turning the pork every 3 to 4 minutes.
Meanwhile, toss the garlic cloves, Brussels sprouts, and shallots in a bowl with the remaining olive oil, and white wine. Season with salt and the chili flakes. Place the vegetables evenly in the bottom of the roasting pan. When the pork is browned on all sides, remove it from the skillet and place it on a rack in the roasting pan. Take care to disperse the vegetables so the heat in the oven can circulate around and under the pork as it cooks. Pour any remaining marinade over the pork.
Finishing and serving the pork: Place the roasting pan in the center of the oven and cook for 35 to 45 minutes. Test the temperature in the center of the loin.
If underdone, cook for 5 to 10 additional minutes and test again. Remove the pork from the pan and allow it to "rest" for 10 minutes. Place the roasting pan on top of the stove. Put the heat on low and add the Sherry vinegar and the remaining lemon zest and juice. Stir to blend with the vegetables. Taste for seasoning, then sprinkle with the breadcrumbs.
Place the pork in the center of a platter (sliced or whole) and spoon the vegetables and any cooking liquid over the top.
The meat can stay in this marinade for a few hours or overnight. You are looking for an "ideal" internal temperature of 125 degrees F. The most important thing? Be patient. Get the other vegetables together as the meat browns.