Brine pork roast 1 day ahead: mix 1/2 cup salt with 1 gallon water. Immerse pork roast into salt water, cover and refrigerate up to 24 hours.
Two hours before serving time: remove roast from brine; pat dry. It's OK to have a thin fat layer for the top of the roast, which will tenderize it. Allow pork to come to room temperature about 1/2 hour while you are making the herb rub.
Make herb rub by combining olive oil, garlic, parsley, rosemary and pepper.
Preheat oven to 350 degrees.
Dice onion; place in bottom of 9x13 pan that has been sprayed with Pam. Cover pork roast on all sides with herb mixture. Place coated roast on top of onions, fat side up.
Roast in 350 oven, covered, for 1/2 hour. Then uncover, insert meat thermometer and roast uncovered until pork hits 155 degrees, about 30 more minutes (it will rise to 160 after you remove it from the oven).
Remove roast from oven - remove all dripping from the pan and keep roast covered with foil while you make gravy.
Gravy: Allow fat to rise to the top of the drippings - discard fat layer. I had about 1/2 cup defatted drippings (the cooked onions add to the flavor so I like to keep them in the gravy). Add drippings to sauce pan. Mix 2 T. cornstarch with 8 ounce canned beef broth and pour into saucepan with drippings. Bring to boil and simmer until gravy is thickened.