Preheat oven to 350°F.  

In a small frypan, cook onion in butter on medium heat setting until translucent, about 4 minutes. Remove from heat.

Combine chicken bouillion and 1/3 cup water to make chicken stock. In a mixing bowl, combine cubed bread, sage and pepper. Add onion and toss. Mix in chicken stock. Set aside.

Wash pork loin. Cut lenghwise to within 1 inch of uncut side, and open the roast like a book. Make balls of the dressing and place them along the length of centre of roast. Pinch the roast edges together while pushing the dressing into the cavity. Using binding string (for roasting), tie the roast every 2 inches to hold the roast together. See below.   

Transfer loin to roasting pan and bake for about 1 hour and 40 minutes or for 25 minutes per pound. Remove from oven to check internal temperature. It should be at least 155 degrees F.  

Let the roast rest for 10 minutes, and then cut and serve.