Preheat your oven to 375°F. Butter a 9x13-inch baking dish generously and set aside. If your bread isn't already dried, spread the cubes on a rimmed baking sheet and bake at 300°F for 20–25 minutes until dry but not browned. Transfer to a very large mixing bowl.

In a large skillet over medium-high heat, cook the Italian sausage, breaking it into small crumbles, until deeply browned and cooked through — about 7–9 minutes. Work in batches if needed to avoid crowding. Using a slotted spoon, transfer the sausage to the bowl with the bread, leaving the drippings in the pan.

Reduce the heat to medium. Add 4 tablespoons of butter to the sausage drippings in the skillet. Once melted, add the diced onion and celery and cook, stirring occasionally, until softened and golden — about 8–10 minutes. Add the garlic and cook 1 minute more until fragrant.

Add the fresh thyme, sage, and parsley to the skillet, stir to combine, and cook for 1 minute. Season with salt, pepper, and red pepper flakes if using. Pour the entire skillet contents — vegetables, herbs, and all the fat — over the bread and sausage in the bowl. Toss gently to combine.

In a separate bowl or large measuring cup, whisk together the warm chicken broth and beaten eggs until fully combined. Season with a pinch of salt. Pour the broth mixture over the bread mixture in a steady stream, tossing gently as you pour so the bread absorbs evenly. The mixture should be moist but not soupy — the bread should hold its shape.

Transfer the stuffing mixture to the prepared baking dish, spreading it into an even layer. Dot the top with the remaining 2 tablespoons of cold butter, cut into small pieces. Cover tightly with aluminum foil.

Bake covered for 35 minutes, then remove the foil and bake for an additional 20–25 minutes until the top is deeply golden and crispy and the center is set. A knife inserted in the center should come out clean with no raw egg.

Remove from the oven and let rest for 5 minutes. Garnish with fresh parsley and serve hot directly from the baking dish.