Preheat oven to 400
Butter a baking dish, ~2 qt
Melt 4 tbsp butter, and add with bread cubes to large mixing bowl and toss
Spread coated bread on baking sheets(s), and bake, stirring occasionally, until dry and has taken on some color. ~15-20 minutes
Return croutons to large mixing bowl
In a large skillet over medium heat, add 2-4 tbsp butter, foam out, add onions, mushrooms, celery and cook until onions are soft, translucent and mushrooms have taken some color
Add 1 tbsp butter to vegetable mixture, add herbs and garlic, stir / toss until fragrant, ~ 1-2 minutes
Add broth to vegetables, season to taste
Add vegetables and broth to croutons, in small increments, and mix well, Looking for a loose dough, not soup. (use all the veggies, but be moderate in adding the broth)
Add egg if used, and mix well
Pour stuffing mix into baking dish, and bake until set, heated through and crust is browned. ~45 minutes
Cool, portion and serve.