Position the racks in the upper and lower thirds of the oven and preheat to 425 degrees . Lightly grease two 12-cup muffin pans. In a large pot, melt the butter over medium. Add the onions, celery, onion and garlic powders, basil, oregano and 1/2 tsp. pepper. Cook, stirring occasionally, until vegetables soften, about 5 minutes. Add the mushrooms and cook, stirring often, until mushrooms are tender and the liquid evaporates, about 7 minutes. Add the stock and bring to a boil.
Remove from the heat and gently stir in the stuffing and buttermilk; season with salt and pepper. Using a fork, stir in the eggs. Cover and let stand 5 minutes. Divide the mixture among the muffin cups.
Bake the muffins until golden, rotating the pans halfway through baking, about 25 minutes. Let cool on a wire rack 15 minutes. Run a thin knife around the edges of the muffins to release them from the pan.