Place butter in a 6.5-inch cast-iron skillet or a 5x5-inch oven-safe baking dish, then put it in the oven to preheat. As the skillet or dish warms, the butter will melt.
In a small bowl, mix together the egg, milk, mustard, salt, and nutmeg.
Whisk in flour thoroughly to avoid any lumps.
Add ham and cheese to the mixture.
With oven mitts, carefully take the skillet or dish out of the oven. Swirl the melted butter around to coat the surface evenly.
Pour the batter into the skillet or dish.
Return it to the oven and bake until the batter puffs up in the center and the edges turn golden brown, which should take about 20 minutes.
Remove from the oven. Use a spatula to gently move the Dutch Baby onto a cooling rack or a plate. Let it cool a bit before serving. Enjoy!
Notes
Smooth Batter: For the best results, aim for a lump-free batter. A hand whisk or electric mixer can ensure that all ingredients are perfectly blended.
Preheat Your Skillet: Warming the skillet in the oven before adding the batter is crucial for that signature puffiness. This simple step ensures your Dutch Baby rises beautifully.
Spice It Your Way: While nutmeg adds a unique flavor, especially when paired with Gruyère, it's totally optional. Customize the seasoning to match your own taste preferences.
Optimal Cookware: Use either a 6.5-inch cast iron skillet or a 5-inch baking dish for the perfect Dutch Baby thickness. If your dish is larger, expect a flatter pancake.
High-Heat Cookware: Make sure your chosen skillet or baking dish is oven-safe up to 400 degrees Fahrenheit. Not all cookware can handle such high heat, so double-check before you start cooking.