For the Brats:

1. Preheat the oven to 400°F and line a large baking sheet with parchment paper. Set aside.
2. Heat a large skillet over medium heat and add the butter. Once the butter is melted, add the onion and a pinch of salt. Cook for a few minutes or until the onion is softened.
3. Add the bratwursts to the pan and allow to brown a bit on each side. Pour the beer over the bratwursts and stir to combine. Bring to a simmer, cover the pan, and cook for 10 minutes or until the bratwursts are cooked through.
4. Remove the bratwursts from the pan and set on a plate – we want them to cool off before wrapping in the pastry.
5. Strain the beer into a measuring cup – discard the onions. Save the beer for the cheese sauce.
6. Place a sheet of puff pastry on a lightly floured counter or cutting board and use a rolling pin to gently roll over the pre-cut seams of the pastry to seal them.
7. Spread the mustard evenly all over the surface of the pastry. Drizzle the honey over the mustard.
8. Using a sharp knife, cut the pastry into four equal squares.
9. Place a bratwurst on the edge of one pastry square. Roll the puff pastry around the bratwurst, pinch the seam closed, and then place seam-side down on the prepared baking sheet. Repeat with the remaining bratwursts.
10. Prick the tops of the rolled bratwursts with a fork and then brush with egg wash. Sprinkle kosher salt over top. If the pastry gets too warm from the heat of the bratwursts, pop the whole tray in the freezer for 10 minutes before baking.
11. Bake for 20 minutes or until golden brown, puffed, and crispy.

For the Beer-Cheese Sauce:

1. While the bratwursts are baking, make the cheese sauce by melting the butter in a medium saucepan over medium heat.
2. Once the butter is melted, add the flour and whisk to combine. Cook the mixture for 1 minute or until it is starting to turn light golden brown.
3. Stream in the reserved beer (you should have about ¾ cup beer left – if more beer cooked off, add some more beer to your measuring cup to equal ¾ cup) from cooking the bratwursts and bring to a simmer.
4. Stir in the milk and heavy cream. Cook the mixture until it is thickened and creamy – enough to coat the back of a spoon.
5. Stir in the grated cheese and mustard and mix until the cheese is completely melted and the sauce is creamy and thick. Season, to taste, with salt and pepper.

To Serve:

Serve the puff pastry bratwursts alongside the cheese sauce for dipping while everything is still hot. You can serve the bratwursts whole or cut them into smaller pieces.