In a medium skillet, add 2 Tbsp olive oil and heat over medium-high. Add the sausage rounds and cook until golden—2-3 minutes per side. Remove from heat and set aside.
Preheat oven to 400 degrees F.
On a lightly floured surface, roll the puff pastry into a 10x16 rectangle; cut into 40 2-inch squares. Some of the squares will have some irregular shapes, so as you make the bites, use a rolling pin to individually roll out the squares a bit thinner and wider if they are too small to hold the fillings.
Spread 1/4 tsp mustard into the center of each square. Top with about 1/2 Tbsp of onion jam, a cube of cheese, and sausage. Pinch the corners into the top center, invert, and place onto another puff pastry square. Bring the four corners up to the center and press down to seal—essentially, you’re wrapping a square on the bottom and top of the fillings to ensure they stay inside (although a little cheese leaking out is fine).
Place the bites onto a large parchment-lined baking sheet and brush with egg wash. Bake on the center rack for 20-25 minutes until browned and golden. Serve warm.
Make the board:
Serve on a plate in the center of a big charcuterie board. Add the veggies, dips, chips, crackers, cheese slices and sweets aroud the plate and serve!