Combine spices in a small bowl and mix. Pat the spice rub all over the pork tenderloin.
Heat butter in a large cast-iron skillet over medium/high heat. Sear pork tenderloin for 2-3 minutes on each side until browned.
Remove pork tenderloin from skillet. Reduce heat to low/medium. Deglaze pan with 1.5 cups of broth, scraping up browned bits.
Add onion, garlic, mushrooms, and balsamic vinegar. Cook for 3 minutes.
Turn off heat. Place pork tenderloin back into the skillet, arranging vegetables around it.
Roast at 400ºF for 10 minutes. Remove from oven, check temperature, flip pork, and bake for another 12-16 minutes, or until internal temperature reaches 145ºF.
Remove pork tenderloin from oven and let it rest on a plate.
Prepare the gravy: Add remaining broth (up to 1 cup) to the skillet over medium/high heat. Whisk in cornstarch until thickened.
Notes
This recipe is easily adaptable. You can add other herbs like rosemary or thyme to the spice rub. For a richer gravy, you can add a splash of heavy cream at the end.