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.