In a very hot wok add oilve oil and butter. Add onions and stir. Continue to stir onions until onions start to turn a dark purple (about 15 minutes). While stiring add brown sugar and balsamic vinegar. Continue to stir until the liquid is reduced to about half. Salt & pepper to taste. Cover and set aside.
Preheat oven to 375 degrees. Rub tenderloin with remaining olive oil. In a very hot saute pan, sear tenderloin on all sides until crispy. Place tenderloin in roasting pan and spread carmelized onion mixture on top. Roast until instant read thermometer reads 160 deg. Remove from oven and tent with foil for 10 minutes.
Carve tenderloin across grain about 3/4" thick and stack around a peak of roasted garlic mashed potatoes. Pour pan juices and onion mixture on top of stack.
Serve with steamed green beans and roasted red peppers.
Garnish with fresh basil.