Preheat smoker to 225°F (107°C). Season the beef tenderloin generously with salt and black pepper. Rub olive oil over the entire surface of the tenderloin. Place the tenderloin in the smoker and smoke for about 1.5 to 2 hours, or until it reaches an internal temperature of 130°F (54°C) for medium-rare. Remove from smoker and let rest for 10 minutes before slicing.

Roasted Mushrooms

Roast the Mushrooms

Preheat the oven to 400°F (200°C). In a bowl, toss the mushrooms with minced garlic, fresh thyme, salt, and olive oil until well coated. Spread them out on a baking sheet in a single layer. Roast in the oven for about 20 minutes, or until tender and slightly browned, stirring halfway through.

Crispy Onions

Prepare the Crispy Onions

Soak the thinly sliced onions in buttermilk for at least 30 minutes. In a separate bowl, mix flour and salt. Heat vegetable oil in a deep fryer or a large pot to 350°F (175°C). Dredge the soaked onions in the flour mixture and fry in batches until golden brown and crispy, about 2-3 minutes per batch. Drain on paper towels and set aside.

Dijon Horseradish Sauce

Make the Sauce

In a small bowl, mix together Dijon mustard, prepared horseradish, and mayonnaise until well combined. Set aside.

Tomato Jam

Prepare the Tomato Jam

In a saucepan, combine diced tomatoes, sugar, apple cider vinegar, and salt. Cook over medium heat, stirring frequently until the mixture thickens, about 20-25 minutes. Let it cool before using.

Sandwich Assembly

Assemble the Sandwich

Slice the smoked beef tenderloin into thin pieces. Toast the sandwich rolls if desired. Spread a layer of dijon horseradish sauce on the bottom half of each roll. Layer with sliced beef tenderloin, roasted mushrooms, YaYa's cheese blend, and crispy onions. Top with a spoonful of tomato jam and finish with the top half of the roll. Serve immediately.