Serving suggestion: assorted mustards.
Preheat an oven to 205°C.
Lightly butter a medium well-seasoned cast iron pan. Put the sausage in the skillet, and bake until almost cooked, about 15 minutes.
Meanwhile, whisk the flour, sugar, and salt together in a medium bowl. In another bowl, whisk the eggs, milk, and mustard until blended. Whisk the liquid ingredients into the dry, just until you have a slightly lumpy thin batter.
Carefully, pour the batter over the partially-cooked sausages and continue to bake until the batter puffs and is crispy, and brown, about 25 to 30 minutes more.
Serve the Toad-in-the-Hole, hot, in the pan with more mustard on the side.
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