In a bowl, thoroughly combine the flour, yeast, baking soda, salt, sesame seeds, and scallions. Add the milk and melted butter and stir to form a dough. Work the dough for a minute or two, until you have a smooth ball. Let rest for 15 minutes.
Preheat the oven to 375 degrees. Divide the dough in half and take out a large piece of parchment or wax paper. Brush some oil on your rolling pin to prevent sticking. On the parchment paper, roll the dough to the thickness of a quarter (about 2 millimeters). The thinner the dough, the crispier the cracker.
Choose whatever shaped cutter you'd like to use to cut the crackers (we just used a small glass), and place them on a baking sheet. Use a fork to poke holes in the crackers, to prevent them from puffing up while baking. Bake the crackers for 12-15 minutes, until golden.