Trim any silverskin or excess fat from the beef tenderloin. Butterfly the tenderloin by carefully slicing it horizontally and open it like a book. Gently pound to an even ¾-inch thickness using a meat mallet between two pieces of parchment.

Season the inside and outside with half of the beef bouillon powder, salt, and pepper.

In a large skillet over medium-low heat, melt 1 tablespoon butter and 1 tablespoon olive oil. Add the thinly sliced onions and cook slowly, stirring regularly until deeply golden and caramelized, about 25–30 minutes.

Season with a pinch of salt and remove from the pan.

In the same skillet, add remaining olive oil and butter. Increase heat to medium-high. Add chopped cremini mushrooms and sauté until they've released their moisture and begin to brown, around 8–10 minutes.

Add minced garlic and cook for 1 minute. Stir in fresh thyme, breadcrumbs, bouillon powder, and mix well. Combine with the caramelized onions and let cool for 5–10 minutes.

Once slightly cooled, fold in grated Gruyère.

Preheat oven to 425°F (220°C). Spread the mushroom and onion mixture evenly onto the butterflied tenderloin, leaving a 1-inch border.

Roll the beef tightly into a log and secure with kitchen twine at 2-inch intervals. Pat the surface dry and season with remaining salt, pepper, and bouillon powder.

Heat a large, oven-safe skillet over medium-high and add a splash of oil. Sear the stuffed tenderloin on all sides until deeply browned, about 2-3 minutes per side.

Remove beef temporarily and pour off any excess fat. Pour red wine into the hot skillet and scrape up any browned bits for maximum flavor. Add beef stock and nestle the stuffed tenderloin back into the pan.

Transfer skillet to the preheated oven and roast for about 20–25 minutes, or until the center reaches 130–135°F for medium-rare.

Remove tenderloin from the oven and transfer to a cutting board. Tent loosely with foil and let rest at least 10–15 minutes before slicing.

Gently simmer pan juices on the stovetop to reduce slightly. Slice the tenderloin into thick rounds and drizzle with the reduction before serving.