Add butter in a very large nonstick pan or cast iron skillet over medium-high heat.
When it's fully melted, add onions, garlic, celery, carrots, mushrooms, salt, pepper, poultry seasoning, Italian seasoning and saute for a few minutes until veggies are partially softened and excess liquid from mushrooms has evaporated.
Remove pan from heat.
Add cornbread, bread cubes and mix until combined.
Pour eggs, 2.5 cups broth and mix until combined. The bread should feel slightly moist. It should not be soaking wet.
Spread the stuffing evenly in the pan.
Note: For better presentation, especially if you are planning on serving this at a Thanksgiving feast, transfer mixture to a large casserole dish (Dimensions: 9x13) and then continue with the remaining steps below.
Top with cubes of butter.
Cover with aluminum foil.
Bake at 350 degrees F 20 minutes.
Remove from oven and discard foil.
Check the consistency of stuffing. If it feels too dry, add more broth. You may have to do this if you prefer a "wet" stuffing.
Bake uncovered for 10-15 minutes or until top is golden brown.
Remove from oven and fluff the dressing with a fork. Enjoy!