Immediately transfer the popcorn to a large serving bowl. Toss the popcorn with the remaining two tablespoons oil. Sprinkle with salt, to taste, and serve.
For the rosemary oil:
Combine the oil and rosemary in a heavy small saucepan. Cook over medium-low heat for about five minutes.
Remove from the heat and leave to cool to room temperature. Transfer the rosemary sprigs to a 220ml bottle. Add the oil and seal the lid. Refrigerate for up to 1 month.