Preheat your oven to 400°F (200°C).

In a large skillet, heat the olive oil and butter over medium heat. Add the sliced onions and fennel, seasoning with salt and pepper. Cook, stirring occasionally, until the vegetables are caramelized, about 20-25 minutes. Add thyme and rosemary, and cook for an additional 5 minutes. Remove from heat and set aside.

Season the veal tenderloin generously with salt and pepper. In the same skillet, sear the tenderloin on all sides until browned, about 2-3 minutes per side.

Transfer the seared tenderloin to a baking dish. Spread the caramelized onions and fennel mixture evenly over the top of the tenderloin.

Place the whole garlic cloves around the tenderloin in the baking dish. Pour the beef broth and red wine into the dish, and drizzle with balsamic vinegar and Dijon mustard.

Roast in the preheated oven for 25-30 minutes, or until the internal temperature of the veal reaches 145°F (63°C) for medium-rare.

Remove the tenderloin from the oven and let it rest for 10 minutes before slicing.

While the veal is resting, place the baking dish on the stovetop over medium heat. Bring the liquid to a simmer and reduce for about 5-10 minutes until slightly thickened. Taste and adjust seasoning if necessary.

Slice the tenderloin and serve it drizzled with the rich roasted garlic gravy. Garnish with fresh parsley.