Using a food processor, combine the butter and cheese. Add the eggs, spinach and seasoning packet from the stuffing. Pulse until combined. Transfer the mixture to a large bowl and fold in the bread packet from the stuffing. Roll into balls, about 1-inch in diameter, and freeze on a cookie sheet.
After a few hours, transfer the frozen spinach balls to a freezer bag and keep in the freezer until ready to bake.
To bake the balls, preheat the oven to 350˚F. Place the balls 1-inch apart on a baking tray. Bake in the oven until cooked through and slightly brown on the bottom, about 15 minutes.
For the sauce: Beat the eggs and half of the sugar together in the top of a double boiler set over gently simmering water. Whisk frequently to ensure the eggs do not become lumpy. In a separate bowl, combine the other half of the sugar and the dry mustard. Add to the eggs and stir until well combined.
Add the vinegar and butter and continue to stir until the sauce has thickened. This can take up to 10 minutes. Once the sauce has thickened, transfer it to a bowl and cover with plastic wrap touching the surface. Cool slightly and then keep in the fridge until ready to use. I find this sauce can be served cool or at room temperature with the cooked spinach balls.
It is critical that the spinach used in this recipe is as dry as possible. I typically leave the spinach to thaw in the fridge overnight. In the morning, place the thawed spinach in a colander and press out as much water as you can. Next, wrap the spinach in an old tea towel and squeeze until all the water has come out.