Preheat oven to 350°. With a rolling pin, roll out puffed pastry sheets. Spread mustard evenly between two sheets of puffed pastry. On top of mustard, place slices of cheddar cheese (make sure to leave puffed pastry free of cheese on one edge so it 'sticks' to the dough when you wrap the sausage). Next, cut kielbasa ring in half (skip this step if using straight sausage). Place on puffed pastry and wrap kielbasa with dough until covered completely around, seal edge by pressing firmly on them. Make sure sausages are wrapped very tightly. Cut excess dough off sides with a pizza cutter of knife. Cut wrapped sausage into bite sized pieces and place on a baking sheet lined with parchment. With a brush, brush beaten egg on top of kielbasa bites. Sprinkle with sesame or poppy seeds and salt. Place in middle rack of oven and cook until golden brown (about 15-20 minutes). Serve immediately while still hot with optional dipping mustard or cheese sauce. Enjoy!