a sous vide machine with sous vide bags or freezer bags
Preheat a sous vide machine in a water bath to 140 degrees F.
Sprinkle the pork tenderloin with salt and pepper. Combine the fennel seeds, garlic, pepperoncini peppers, rosemary and 1/4 cup olive oil in a blender and blend until smooth. Rub the marinade over the pork, then place the pork into a sous vide or food-safe freezer bag. Remove the air from the bag and seal. Place the pork in the sous vide bath and cook for 1 hour 30 minutes. Remove the pork from the bag and allow the pork to rest for 10 minutes.
Heat a grill, large saute pan or grill pan over high heat. Add the pork and sear on all sides until browned, 5 to 6 minutes total. Set aside.
Combine the apricot jam, brown mustard and pickle juice in a bowl. Season with salt and pepper and set aside.
Slice the baguette in half lengthwise. Brush the cut side of the baguette with the remaining 3 tablespoons olive oil and sprinkle with salt and pepper. Grill until
lightly toasted, 2 to 3 minutes. Spread the mustard mixture over the inside of the baguette. Slice the pork into thin slices and arrange on the bottom half of the baguette. Top with the red onions and arugula, followed by the top half of the baguette. Slice into sandwich-size pieces, 3 to 4 inches long.