Remove the pork loin from the refrigerator half an hour before cooking to come to room temperature. Preheat the oven to 500°.
In a mortar and pestle, combine the fennel and mustard seed. Crush them until they're evenly mashed. Transfer to a small bowl and add the salt, pepper, thyme and rosemary. Mix to combine and set aside.
Place the pork loin in a roasting pan and drizzle with 1 1/2 teaspoons olive oil. Spread the olive oil over the surface and sides of the pork. Sprinkle with about 2/3 of the spice mixture, pressing into the flesh so that it adheres.
Place the pork loin in the hot oven and roast for 20 minutes. While the roast is cooking, add the potatoes, carrots and onions to a large bowl. Drizzle the remaining olive oil over the potatoes and add the remainder of the spice blend. Toss to coat.
After the pork has roasted for 20 minutes, shake the pan to make sure it hasn't stuck to the bottom. Pour the vegetables around the pork roast in an even layer. Roast the pork and vegetables for an additional 15-20 minutes or until the internal temperature of the pork reaches 140°.
Remove the pork from the oven and transfer the roast to a cutting board. Tent with foil and rest for 10 minutes before carving.
Continue to roast the vegetables for an additional 10 minutes until tender and golden. Transfer the vegetables to a serving platter with a slotted spoon, draining off any excess fat back into the roasting pan.
Make the Pan Gravy:
Place the roasting pan on the stove top and heat to medium high heat. Sprinkle the flour over the fat and stir until the flour and fat are incorporated. Stir in the chicken broth, stirring constantly and bring the sauce to a boil. Cook for one minute, until slightly thickened. Stir in the dijon and vinegar until well blended. Taste for seasonings and add salt and pepper as needed.
Carve the roast into 1/4" - 1/3" slices and transfer to the serving platter with the vegetables. Serve with the pan gravy.