Ham and Gruyère Popover Sandwiches

Instructions

1. Prepare the batter
   In a blender, combine the eggs, milk, flour, salt, pepper, and mustard powder. Blend on high speed for 30 seconds until smooth. Add the melted butter and blend for another 10 seconds. Let the batter rest at room temperature for 15 minutes.
2. Preheat the oven
   Place a 12-cup popover pan or muffin tin in the oven and preheat to 450°F (230°C). Allow the pan to heat for at least 10 minutes.
3. Prepare the filling
   While the batter rests and the oven preheats, tear or cut the ham into small pieces. Mix the ham with 1 cup of the grated Gruyère and the chopped chives in a bowl. Set aside.
4. Bake the popovers
   Carefully remove the hot pan from the oven and quickly brush each cup with butter. Fill each cup about 1/3 full with batter, add a spoonful of the ham and cheese mixture, then cover with more batter until the cup is about 3/4 full. Sprinkle the remaining Gruyère on top. Return the pan to the oven and bake for 20 minutes, then reduce the heat to 350°F (175°C) and bake for an additional 5 minutes until golden brown and puffed.
5. Cool briefly
   Remove the popovers from the oven and let them cool in the pan for 2-3 minutes. They will deflate slightly as they cool, which is normal.
6. Serve
   Gently remove the popovers from the pan. Spread a thin layer of Dijon mustard inside each popover if desired, add a few leaves of baby arugula, and serve immediately while still warm.