Heat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Brown the sausage in a large skillet over medium heat until crumbled. Spoon off excess fat.
Stir in olive oil and the sautéing butter. Cook onion with a pinch of salt until translucent, 3–4 minutes. Add garlic; cook 30 seconds.
Deglaze with broth, scraping up browned bits, and simmer 1 minute.
Add mushrooms. Season with Italian seasoning, salt, pepper, and red pepper flakes. Cook until they release liquid and reduce, 8–10 minutes.
Lower heat. Mix in cream cheese until melted and creamy. Fold in Worcestershire, half the Parmesan, and half the parsley. Adjust seasoning.
Spread mixture in the baking dish. Toss panko with melted butter. Top casserole with mozzarella, remaining Parmesan, then the buttered panko.
Bake 18–22 minutes until bubbly and golden. Broil 1–2 minutes if needed. Rest 5 minutes, then finish with remaining parsley and lemon zest.