Preheat an oven to 400 degrees F.
Lightly butter a medium well-seasoned cast iron skillet. 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 skillet with more mustard on the side.
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