Preheat the Oven: Preheat your oven to 350°F (175°C). Grease an 11 x 13-inch baking dish with butter.
Prepare the Bread Stuffing: Place the unseasoned bread stuffing in a large mixing bowl.
Cook the Veggies: In a large skillet, melt the butter over medium-high heat; add the onion, red bell pepper, and celery. Cook for about 8 to 10 minutes or until the vegetables soften. Add the minced garlic and cook for an additional 2 minutes. Add the sliced mushrooms and cook until they start to brown and their liquid evaporates about 5-8 minutes.
Add The Herbs: Stir in the chopped sage, rosemary, thyme, and cilantro— season with ground black pepper to taste. Transfer the cooked vegetable mixture to the bowl with the bread stuffing, making sure to scrape out every last bit from the pan.
Cook the Sausage: In the same pan, cook the spicy Italian sausage over medium-high heat for 8-10 minutes or until it's browned and thoroughly cooked. Use a metal spatula to break up the sausage into pieces no larger than ¼-inch.
Combine Sausage and Bread Mixture: Add the cooked sausage (and any fat from the pan) to the bread stuffing and vegetables. Gently toss to combine.
Prepare the Liquid Mixture: In a separate bowl, whisk together the egg, water, and Knorr Granulated chicken flavor until well combined. Gradually pour this liquid mixture into the bread, sausage, and vegetable mixture. Lightly toss everything together to combine, being careful not to break up the bread too much. This step helps to distribute the moisture and flavor evenly throughout the stuffing.
Bake the Stuffing: Transfer everything to the prepared baking dish. Bake in the preheated oven for 65-75 minutes or until the top is golden and crispy.
Serve: Once done, let it cool slightly before serving.