Marinate pork in Italian dressing, at least 30 minutes, preferably overnight.
Preheat oven to 350 degrees Fahrenheit.
Remove pork from marinade and discard marinade.
Pat dry and season well with salt and pepper.
Heat 2 tbsp butter in a large heavy pan.
Brown pork loin on all sides, about 2 minutes per side.
When pork loin has browned, add to roasting pan or baking sheet.
Melt rest of butter in pan and combine onion, herbs and garlic - heat 30 seconds and remove from heat.
Wash potatoes well and cut in half. Pat dry.
Arrange around pork loin in pan.
Generously season pork and potatoes with herbs, salt, and pepper.
Drizzle butter and herbs over pork and potatoes. Toss potatoes lightly to coat in herb butter.
Roast 35-40 minutes, checking temperature after 20 minutes.
When porks internal temperature reaches 140 degrees, remove from oven and let rest 10 minutes before carving.
Serve and enjoy!