Preheat the oven to 350 degrees F.

In a large saute pan over medium heat, melt the butter. Add the diced onions and bell peppers and cook while stirring occasionally until soft but not brown, about 3 to 4 minutes. Add the garlic and mushrooms and saute for another 2 to 3 minutes.

Sprinkle the flour over top and stir vigorously while it gets absorbed by the mushrooms and browns, about 1 to 2 minutes. Be careful not to let the flour burn. Pour in the milk, broth, and salt and cook while scraping the brown bits off the bottom of the pan until the liquid thickens to the consistency of gravy, about 1 to 2 minutes. Turn off the heat and fold in the bread cubes until evenly coated.

Remove the pan from the stove and let it cool for a few minutes. Stir in the eggs and then transfer the whole mixture to a rectangular baking dish (or stuff it inside the turkey). If cooking as a separate casserole bake until golden brown on top, about 40 minutes.