Prepare puff pastry, make sure it's thawed, the kind I purchased requires 40 min of thaw time. Unfold puff pastry carefully and using a large circle cookie cutter cut out circles and lay on a cookie sheet that has a silpat on it (or parchment paper). Brush tops with 1TB melted butter. Set aside until oven is done pre-heating, when oven is ready bake for 20-25 min, or until the puff pastry is golden brown and puffy. While pastry is baking, prepare chicken filling.
Melt the remaining 2 TB butter in a large, skillet over medium-high heat. Add the onion and cook until translucent, about 5-7 minutes. Add the flour and stir to incorporate. IN a small bowl or measuring cup (capable of holding at least 2 cups of liquid) whisk together the milk and chicken stock. Pour the chicken-milk mixture in a thin stream into the skillet, whisking constantly. Bring to a simmer to thicken.
Adapted from i am a food blog. Prep time includes defrosting of puff pastry.