Preheat oven to 425°F/220°C with rack in the center of the oven.

To a large bowl with a spout. Add the flour, baking powder and salt and eggs. Whisk the eggs until combined, then while whisking drizzle in the milk. Whisk well to combine. Stir in the rosemary, set aside.

To a 9 x 13-inch/ 23 x 33 cm casserole dish, add the oil. Place the pan in the oven to heat for 10 minutes.

After 10 minutes add the sausages and bake for 10 minutes. Turn the sausages. They don't have to brown, they will brown once the batter is added.

Slide the rack out of the oven and  pour the batter around the sausages.

Return the pan back to the oven and bake for 30  minutes until the batter is risen and brown.

To a sauté pan, add the butter.  When the butter is melted add the onions and cook, stirring often until softened. 

Whisk the flour into the butter and onions and cook for 1 minute. Whisk in the beef stock, English mustard and Worcestershire sauce until well mixed and bring to a low simmer until thickened. Taste and add salt if needed (this depends on how much salt is in your beef stock).

Cut the toad in the hole into 4 pieces and serve drizzled with the gravy and your favorite side dish.