I love this recipe because it elevates a lean cut of pork and makes it rich and tender.
Whenever I roast a whole cut of meat, I also love to include "built-in" vegetables. These vegetables end up with a wonderful flavor after cooking in the oven. It is important, however, to make sure the pork roast has enough air circulating around (and underneath it) as it cooks. Use a roasting pan fitted with a grill rack so the meat is slightly elevated as it cooks.
Preheat the oven to 500 degrees F.
In a medium bowl, add the Dijon mustard, grainy mustard, the zest and juice from 1 of the lemons, the fresh thyme and the butter. Whisk together and set aside. Heat a large cast iron skillet over high heat and add 2 tablespoons extra-virgin olive oil to the pan. Season the meat on all sides with salt and pepper, to taste, and gingerly put the pork loin in the hot oil. Brown the pork, without moving it around, for 3 minutes. Use a pair of kitchen tongs or 2 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. The most important thing? Be patient. Get the other vegetables together as the meat browns.
For your "Built-In Vegetables":
Meanwhile, toss the garlic cloves, Brussels sprouts and shallots in a bowl with the remaining 2 tablespoons of extra-virgin olive oil. Season with salt, to taste, and stir in the chili flakes. Heat a roasting pan over medium-high heat. Arrange the vegetables evenly in the bottom of the roasting pan and cook just until the vegetables begin to caramelize, about 5 minutes.
When the pork is browned on all sides, remove it from the skillet and put it on a rack in the roasting pan, flesh side up, fatty side down.
In the same cast iron pan that was used to brown the pork, discard some of the leftover fat, leaving about 1 to 2 tablespoons in the pan. Add the diced bread and toast over low heat. Remove to a paper towel lined plate and set aside until service.
Take care to disperse the vegetables so the heat in the oven can circulate around and under the pork as it cooks. Pour the mustard mixture over the pork. Turn the oven temperature down to 350 degrees F and put the roasting pan in the center of the oven and roast for 30 minutes. Test the temperature in the center of the loin. You are looking for an "ideal" internal temperature of 90 degrees F on an instant-read thermometer. Roast for 10 to 15 additional minutes and test again. You are looking for an ideal temperature of 125 degrees F. Remove the pork from the pan and allow it to "rest" for 10 minutes.
Put the roasting pan on top of the stove, over low heat, and add the vinegar and the juice and zest from the remaining lemon. Stir to blend with the vegetables. Taste for seasoning.
Arrange the pork in the center of a platter (sliced or whole) and spoon the vegetables and any cooking liquid over the top. Garnish with croutons and serve.