Melt the butter in a large, heavy-bottomed saucepan over medium-high heat. Add onion and ½ tsp 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 broth in a slow stream. Be careful, it will bubble up quickly.
Stir in the rosemary and pepper and turn the heat down to low. Whisk in cream and remaining ½ tsp salt. When the mixture thickens, turn off heat and cover. PRO TIP: Let stand for 1 hour to infuse flavours.
Just before serving, return the gravy to a simmer over low heat. For silky-smooth gravy, puree it with an immersion blender or force it through a fine-mesh sieve into a serving dish and discard solids.
Notes
Shown here served with a boneless turkey breast roast.