Preheat a rotisserie, grill, or grill pan.

Season tri-tip with salt and pepper. Cook meat until medium-rare on rotisserie or grill, about 20 minutes depending on temperature of cooking medium. Meanwhile, heat a small saute pan over medium-high heat. Season foie gras with salt and pepper, and then sear in hot pan until desired degree of doneness. Slice baguette in half lengthwise and then toast bread, if desired. Spread Peach Mustard over bottom piece of bread. Slice tri-tip, as desired, and place on bottom half of baguette on top of mustard. Place foie gras on top of tri-tip and then add a dollop of Peach Mustard on top of foie gras.

In a bowl, mix arugula greens, shaved truffles, truffle oil, salt and pepper, to taste. Add to sandwich on top of foie gras. Spread Peach Mustard on top half of baguette, slice, and serve.

In a medium saucepan, cook peaches with brown sugar over medium heat until caramelized. Add rice wine vinegar and stir to combine. When ready to serve, stir in 1 cup caramelized onions, or more to taste. You can vary the amount of onions based on your preference.

For 1 serving, combine about 3 to 4 tablespoons of the peach-onion mixture with about 1 tablespoon Dijon in a small bowl. More or less mustard can be added, to taste. Spread on baguette.

Store extra Peach Mustard in an airtight container in the refrigerator.