Grease 3 quart or 9x13 casserole with butter. In a large stock pot or dutch oven, melt butter over medium-low heat. Add in chopped fresh herbs and sauté until butter is golden brown and very fragrant, about 8-10 minutes.
Add in the diced onions, sauté for 4-5 minutes. Add diced celery, sauté for 4-5 minutes. Add in sliced leeks (be sure to clean well first by soaking in water) and mushrooms, sauté for another 8-10 minutes or until all veggies are cooked through and tender. Add minced garlic and cook 1 additional minute. Stir in 4 cups of chicken broth (reserve 1 cup) increase heat to medium-high and bring to a boil. Remove from heat. Season with salt & pepper to taste.
Gently fold in dry bread cubes and continue to stir until all are coated well. Bread cubes should be soaked with broth (without broth pooling in bottom of pan). Add additional cup of reserved broth if needed. I always go for more moist than dry.
Transfer stuffing to greased baking dish. Bake at 350° F. for 45 minutes or until edges and top are crispy deep golden brown and center is hot. If making ahead, cover tightly and refrigerate for up to 2 days. Allow to come to rest at room temperature for at least 30 minutes before baking.