• Preheat oven to 375˚F (190˚C).
• Sprinkle 1 teaspoon salt and ½ teaspoon ground pepper all over pork loin.
• Brush grill pan with 1 tablespoon vegetable oil. Sear pork loin about 3 to 4 minutes on each side.
• Combine fennel (or carrots), baby potatoes, onion (if using), 1 teaspoon salt, ½ teaspoon dried thyme, and 2 tablespoons vegetable oil in a large pan. Mix well.
  
• Place seasoned vegetables all around the pork loin in grill pan.
• Brush remaining 1 tablespoon vegetable oil on the top of pork loin. Pat ground pistachio evenly over the top of pork loin.
  
• Transfer to preheated oven and roast for 50 minutes or until internal temperature is 150˚F (65˚C).
• Remove and allow pork loin to rest for 10 minutes before carving and serving with creamy horseradish sauce.
  

Creamy Horseradish Sauce

• Melt butter in a small saucepan. Add all-purpose flour. Stir for 2 to 3 minutes until light brown. Pour in milk whisking vigorously to dissolve the flour into the milk until smooth.
  
• Stir in horseradish, salt, and pepper. Cook until sauce is thick and bubbly. Turn off stove.