Preheat your oven to 400 degrees F (200 degrees C).
In a small bowl, mix together the Dijon Mustard (1 tablespoon), Garlic (4 clove), Fennel Seeds (1 teaspoon), and Dried Thyme (1 teaspoon). Then set aside.
Take the Pork Tenderloin (1) out of the fridge around 30 minutes prior to cooking, and let it get to room temperature.
Once at room temperature, liberally season all sides of the pork with Salt (to taste) and Ground Black Pepper (to taste). Once seasoned, rub the Dijon mustard mixture all over the pork.
Set a cast-iron skillet, or sauté pan big enough to hold the pork, over medium-high heat and drizzle with Olive Oil (to taste). Once hot, lower the heat to medium and add the pork pressing it against the pan. Sear all four sides of the pork for 2 to 3 minutes per side, or until browned.
Once all four sides are seared, transfer the pork to a baking sheet and then to the oven. Cook the pork until the internal temperature reaches 145 degrees F (65 degrees C), about 20 minutes. When the pork is done cooking, remove it from the oven, tent it with aluminum foil, and let it rest for at least 5 to 10 minutes.
Over medium heat, add Butter (2 tablespoon) and 1 tbsp of Olive Oil (to taste) to the same pan you cooked the pork in. Once hot, add the Sweet Onion (1) and a pinch of Salt (to taste). Cook the onions for about 10 minutes, or until they get nice and caramelized.
Once caramelized, add the Mushroom (10 ounce) and another pinch of Salt (to taste). Let the mushrooms cook for about 5 minutes or until soft.
While the mushrooms are just about softened, add the remaining Butter (2 tablespoon), along with the Garlic (4 clove), Dried Thyme (1 tablespoon), Dried Oregano (1 tablespoon), and Crushed Red Pepper Flakes (1 teaspoon). Cook for an additional 1 to 2 minutes, or just until all the butter is melted.
Add your All-Purpose Flour (2 tablespoon), and cook for another 2 to 3 minutes stirring continuously.
Once the flour is fully incorporated and cooked off, deglaze the pan with Balsamic Vinegar (2 tablespoon) and mix until combined. Cook for an additional 1 to 2 minutes, scraping all the brown bits off the bottom of the pan.
Turn the head up to medium-high and add the Chicken Stock (2 1/2 cup). Stir until everything is combined and let the gravy come up to a boil. Once boiling, reduce the heat to medium-low and allow the gravy to simmer for 5 to 10 minutes, or until desired consistency.
Serve pork tenderloin with gravy, and enjoy!