Preheat your Oven to 400°F. Line a large Baking Sheet with Parchment Paper or lightly grease it with Cooking Spray.

Wipe the 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved clean with a damp paper towel — do not rinse them under water or they'll become soggy. Gently twist or pull out the stems and set them aside. Lightly brush the outside of each 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved cap with 2 tbsp olive oil divided and place them cavity-side up on the prepared Baking Sheet.

Finely chop the reserved 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved stems. Heat 1 tablespoon of 2 tbsp olive oil divided in a Skillet over medium heat. Add the ½ lb Italian sausage casings removed and cook, breaking it apart with a Wooden Spoon, until browned and cooked through, about 5–6 minutes. Use a Slotted Spoon to transfer the ½ lb Italian sausage casings removed to a plate, leaving the drippings in the pan.

In the same Skillet over medium heat, add the diced 1 small yellow onion finely diced and chopped 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved stems. Cook, stirring occasionally, until softened and any moisture has evaporated, about 4–5 minutes. Add the 4 cloves garlic minced and cook for 1 more minute until fragrant. Remove from heat and let cool for 5 minutes.

In a Mixing Bowl, combine the cooked ½ lb Italian sausage casings removed, sautéed 1 small yellow onion finely diced-24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved mixture, softened 4 oz cream cheese softened, ½ cup Parmesan cheese freshly grated, plus more for topping, ¼ cup Italian breadcrumbs, 2 tbsp fresh parsley finely chopped, plus more for garnish, ½ tsp kosher salt, ¼ tsp black pepper freshly ground, and ¼ tsp crushed red pepper flakes (if using). Stir together until everything is well incorporated and the mixture holds together. Taste and adjust seasoning if needed.

Using a small Wooden Spoon, generously mound the filling into each 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved cap, pressing it in lightly and doming it slightly above the rim. Don't be shy — pack it in! Arrange all filled caps on the Baking Sheet.

Sprinkle the tops of the stuffed 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved with the remaining ½ cup Parmesan cheese freshly grated, plus more for topping and finish with a light drizzle of 2 tbsp olive oil divided. This helps the tops turn golden and slightly crispy.

Bake in the preheated Oven for 20–22 minutes, until the 24 whole button mushrooms large, about 1.5–2 inches wide, stems removed and reserved are tender, the filling is heated through, and the tops are golden brown. If you want extra color on top, Broiler for the last 1–2 minutes — watch closely.

Remove from the Oven and let rest for 3–4 minutes before serving — the filling will be very hot. Transfer to a serving platter, garnish with fresh chopped 2 tbsp fresh parsley finely chopped, plus more for garnish, and serve warm.