Preheat oven to 400°F. Grease a baking dish or roasting pan.
In a small bowl, stir together mustard, honey, and thyme or rosemary.
Place sliced onion in a single layer in the bottom of the prepared dish. Pat the pork dry. Season with salt and pepper on all sides, and then place on top of the sliced onions in the dish. Brush about half of the mustard mixture over the pork.
Cover with foil and bake for 25 minutes.
Turn the oven setting to "BROIL" on high.
Remove the cover, brush the pork with additional mustard sauce, and place about 4-6 inches under the broiler for about 5 minutes, or until a crispy crust forms on top and the pork reaches an internal temperature of 140-150°F.
Remove the dish from the oven and allow the pork to rest for about 10 minutes before slicing and serving. Garnish with additional fresh herbs, if desired.
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Notes
Thesalt and honey are necessary for seasoning the meat and for encouraging caramelization on the outside of the pork.
Allow the pork to sit on the counter and come to room temperature for about 30 minutes before roasting. This will help the meat cook evenly.
The total cooking time may vary, depending on the size, thickness, and temperature of your pork when it goes in the oven. As a result, use an instant read thermometer to know exactly when your pork reaches the desired level of doneness.
Let the pork rest for at least 5-10 minutes before slicing and serving. This will give the juices a chance to redistribute so that your meat is as juicy as possible.