Preheat oven to 425 F. Melt butter in 2.5 quart oval casserole dish ( 13 inches * 9 inches *4 inches deep). Trim the tenderloin of any silver skin (optional).

Scatter sliced onions on the bottom of the casserole dish (where you melted butter). Place sliced plums and apples over the onions.

Generously sprinkle Montreal seasoning (or salt and pepper, or other favorite rub) over the top of the pork tenderloin and fruit. Place pork tenderloin on top of the fruit into the casserole dish. Make sure the seasoning stays on top of the tenderloin.

Loosely cover the casserole pan with aluminum foil and bake for 30-40 minutes in a preheated oven at 425 F, until the tenderloin is cooked through and pale pink in the center, or until a thermometer inserted into the center of the pork reads at least 145 degrees F (63 degrees C).

Depending on your oven, you might have to cook pork longer than 30-40 minutes. Remove the casserole dish with the pork tenderloin from the oven and slice the pork in the center to check for desired doneness - if it's too pink for you, cover the pan tightly with foil and let it sit for 10-15 minutes on the counter.

To serve, slice the pork into 1-inch thick slices, and serve on top of cooked apples and plums.