Melt the butter in a large, heavy-bottomed saucepan over medium-high heat. Add onion and ½ tsp coarse salt and cook until soft, about 5 minutes.
Stir in flour and cook, stirring frequently, until it turns butterscotch colour.
Very carefully whisk in the chicken stock in a slow stream. Be careful, it will bubble up quickly.
Stir in the rosemary and peppercorns and turn the heat down to low. Whisk in cream and remaining ½ tsp salt. When the mixture thickens, turn off heat and cover. Let stand for 1 hour to infuse flavours.
Just before serving, return the gravy to a simmer over low heat. Force through a fine-mesh sieve into a serving dish. Discard solids.