Preheat oven to 375 degrees. If grilling, turn gas grill to high or start charcoal and heat until grill is 375 degrees.
Generously season pork tenderloin with salt and pepper. (UNLESS YOUR PORK IS PRE-BRINED OR DRY BRINED. DO NOT ADD EXTRA SALT IF IT IS.)
In a large, heavy pan, heat olive oil until it is shimmery.
Brown pork tenderloin on all sides - about 2-3 minutes per side.
While pork browns, make gravy by combining melted butter, ranch dressing mix packet, and the juice of one lemon. Stir well to dissolve ranch dressing packet powder.
Lay a sheet of foil on a baking sheet and place pork tenderloin in pouch.
Drizzle ranch dressing gravy over pork tenderloin.
Close foil pouch and bake for 30 minutes, checking temperature with a digital meat thermometer at about 25 minutes.
When pork has reached 145 degrees internal temperature, remove from oven, keeping foil pouch closed.
Let rest for at least 10 minutes to continue carryover cooking to the newly recommended 150 degrees internal temperature for pork.
Garnish with cilantro, parsley, or green onions.
Slice into thin slices and serve immediately or store in an airtight container for lunch meal prep.