Begin by melting the butter in a skillet over medium heat.
Add the onion and celery and cook, stirring frequently, for about 5 minutes until the veggies start to soften. Lower the heat to medium-low if the veggies are browning too quickly. Season with salt and pepper.
Remove the cooked veggies to a heatproof bowl. To the same bowl, add the croutons, beaten eggs, turkey broth, sage, thyme, and parsley. Mix to combine fully. Allow the custard to soak into the bread for about 5-10 minutes.
Preheat the oven to 350 degrees F.
Grease a muffin in. Fill each with about ⅔ cup to ½ cup of the stuffing mixture. The stuffing should come up over the sides of the pan, just like a real muffin.
Bake in a preheated oven for 25-30 minutes until golden brown on top and set in the middle.
Allow the muffins to cool slightly before removing them from the pan. Use a small knife to loosen the sides if needed.
Serve with your other gluten-free Thanksgiving foods.