Heat oven to 240C/220C fan/gas 9. In a food processor, combine the flour, eggs, milk, mustard and some salt and pepper, blitz until smooth, then leave to rest for 30 mins.
Pour the oil into a metal roasting tin about 30 x 23cm and 7.5cm deep. Brush the oil all over the sides and bottom, then place in the oven. When the roasting tin is very hot and smoking, place the sausages inside, evenly spread out, and cook for 5 mins.
Give the rested batter a stir and pour into the really hot tin - take care as it may spit. Quickly sprinkle over the sage leaves and rosemary, then place in the middle of the oven. Do not open the door for 25 mins, then check - if needed, cook for a further 5-10 mins. Cook until puffed up and brown and the batter is completely cooked through. Serve straight from the dish.