Preheat oven to 400°F. Drizzle the oil over the bottom of a cast-iron skillet or 8-inch by 8-inch square baking dish. Roll the sausages around in the oil and arrange over the bottom of the pan. Roast for 10 minutes while preparing the batter.Add the milk, egg, English mustard, flour, and salt to the work carafe of a blender. Fix the lid in place and blend on low until the mixture is smooth. Stir in the fresh thyme leaves. When the sausages have roasted for 10 minutes, pull the pan from the oven and pour the batter in around the sausages. Return the pan to the oven and bake for about 40 to 50 minutes, or until the edges are crisp, the batter is completely set, and the top is deep golden brown with mahogany brown sausages. If you insert the tip of a sharp knife in the center of the Toad in the Hole, it should come out clean without any batter sticking to it. Allow to cool for 3 to 5 minutes before slicing into wedges.
To Make the Onion Gravy:
Melt the butter in a heavy-bottomed saucepan over low heat. Toss in the onions with the kosher salt and soften the onions for about 20 minutes, stirring frequently, until they are golden brown. Sprinkle the brown sugar over the top and cook for 5 minutes more. Sprinkle the flour over the onions and stir until the flour is fully coating the onions and there are no lumps yet. Cook until it smells slightly nutty, then whisk in the browning sauce, followed by slowly drizzling in the broth or stock, whisking constantly until smooth. Bring to a boil and then lower the heat to simmer for 4 to 5 minutes, or until thick and smooth. Serve spooned over hot wedges of Toad in the Hole.
Nutritional information is an estimate and provided to you as a courtesy. You should calculate the nutritional information with the actual ingredients used in your recipe using your preferred nutrition calculator.