Preheat oven to 220°C.
Whisk together salt, flour and pepper in a large bowl, set aside.
Whisk eggs and milk together in another bowl until well combined.
Gradually whisk half the milk mixture into the flour mixture until smooth. Whisk in remaining milk mixture until thoroughly combined.
Set batter aside at room temperature for at least 30-60 minutes. This lets the gluten relax and the starch granules swell, resulting in a much higher puff.
Heat oil in a 12-inch cast-iron skillet over medium heat. Add 1-2 tbsp vegetable oil. Add sausages and cook, turning once, until browned, about 2 to 3 minutes per side.
Once the sausages are browned and the pan is smoking slightly, arrange the sausages in the middle of the batter spaced 1 inch apart.
Quickly pour the batter into the hot pan around the sausages. It should sizzle and start to set at the edges immediately. Bake for 25 - 30 minutes until puffed, lightly browned, and crisp around the edges. DO NOT OPEN THE OVEN for at least 20 minutes.
The toad in the hole is done when it's deep golden brown and has pulled away slightly from the sides of the pan. Remove from the oven and serve immediately with warm onion gravy.
