Step 1: Prepare your toast. Lightly toast the bread slices. You want them to be golden and firm enough to hold the sauce, but not overly crispy. Set aside.
Step 2: Melt the butter. In a medium saucepan or skillet over medium heat, melt the unsalted butter.
Step 3: Make a roux. Add the all-purpose flour to the melted butter, whisking constantly for 1-2 minutes until a smooth paste (roux) forms and cooks slightly. This eliminates the raw flour taste.
Step 4: Gradually add liquids. Slowly whisk in the warm milk, a little at a time, until completely smooth. Then, gradually whisk in the ale or beer. Continue to whisk until the mixture thickens slightly and begins to bubble.
Step 5: Add seasonings. Remove the saucepan from the heat. Stir in the Dijon mustard, Worcestershire sauce, and cayenne pepper (if using).
Step 6: Incorporate the cheese. Add the freshly grated sharp cheddar cheese to the sauce, a handful at a time, stirring continuously until the cheese is fully melted and the sauce is smooth and creamy. Do not let the sauce boil once the cheese is added, as this can make the cheese separate or become oily. Season with salt and freshly ground black pepper to taste. You now have a perfect Welsh Rarebit Recipe sauce!
Step 7: Assemble and broil. Ladle a generous amount of the warm cheese sauce over each slice of toasted bread, ensuring the bread is fully covered. Place the rarebit under a preheated broiler for 2-4 minutes, or until the cheese is bubbling, golden brown, and slightly caramelized around the edges. Keep a close eye on it, as it can burn quickly.
Step 8: Serve immediately. Carefully remove the rarebit from the broiler and serve hot. Enjoy your homemade Welsh Rarebit Recipe!