For marinated pork: Combine orange juice, lime juice, lemon juice, minced garlic, vegetable oil, oregano, black pepper, and salt in a large bowl and stir to combine. Place pork tenderloin in marinade and let sit for 12-24 hours in the refrigerator, flipping once or twice.
Heat charcoal or gas grill to medium-high heat. Lightly oil grates with remaining tablespoon of vegetable oil, and place the pork tenderloin on the grill. Cover and cook for 12-15 minutes, turning every 2-3 minutes, until the tenderloin reaches an internal temperature of 140º F. Sprinkle a little salt onto the onions, and cook for 3-5 minutes per side, until lightly charred and tender. Remove pork from grill and wrap tightly in aluminum foil. Let sit for 10 minutes; then slice. Set aside onions and pork and keep warm until ready to use.
For pickled mustard seeds: Combine mustard seeds, champagne vinegar, water, sugar, salt and crushed red pepper flakes in medium saucepan and bring to a gentle boil. Reduce to a low simmer and let cook for 5 minutes, stirring occasionally. Remove from heat, cover and let cool. Place in refrigerator until ready to use (can be made up to a day in advance). Serve at room temperature.
For sandwich assembly: Place a slice of swiss on four slices of bread, followed by an even layer of pickles, grilled onions, sliced pork, bacon and another slice of swiss. Spread an even layer of pickled mustard seeds across the remaining slices of bread and place on top.
Heat 1 tablespoon of the butter in a large pan over medium-low heat. Add sandwiches to pan and cook for 2-3 minutes, lightly pressing down to gently flatten, until bread is golden brown and slightly crunchy. Melt the remaining tablespoon of butter in the pan, then flip the sandwiches and cook for another 2-3 minutes, gently flattening, until the bread is golden brown and crunchy, and the cheese is melted. Remove from grill and serve hot.
For Marinated Pork:
- 1 cup orange juice
- 1/4 cup lime juice
- 1/4 cup lemon juice
- 8-10 cloves garlic, minced
- 2 tablespoons vegetable oil
- 2 teaspoons dried oregano
- 1 1/2 teaspoons salt
- 1/2 teaspoon freshly ground black pepper
- 1 pound pork tenderloin
For Pickled Mustard Seeds:
- 1/3 cup yellow mustard seeds
- 1/2 cup champagne vinegar
- 2 tablespoons water
- 1 tablespoon sugar
- 1/2 teaspoon salt
- 1/2 teaspoon crushed red pepper flakes (optional)
For Sandwich Assembly:
- 8 slices (1 ounce each) Wisconsin Swiss cheese
- 8 thick slices challah bread
- 1 cup bread and butter pickles
- 1 large sweet onion, sliced and grilled
- Prepared marinated pork
- 8 slices cooked bacon
- Prepared pickled mustard seeds
- 2 tablespoons salted butter