Make the Vegan Alfredo Sauce and set aside. This can also be made the day before and stored in the fridge.
Preheat the oven to 400 degrees fahrenheit.
Heat the olive oil in a large sauté pan over medium heat. Add the sliced mushrooms and diced onion and stir for 3 minutes. Add the broccoli florets (make sure they’re in bite-sized pieces and not too large) and stir for another 3 minutes or until the broccoli is crisp-tender and bright green.
Place the shredded chicken in a 9×13-inch casserole dish, covering the bottom. Add the sautéed mushrooms, onions and broccoli in a flat layer on top.
Stir the chicken broth into the Vegan Alfredo sauce, then pour on top of the broccoli in the casserole dish. Use a spatula if needed to make sure everything is well coated.
Cook the casserole for 20-25 minutes. The sauce should be bubbling and slightly reduced and thickened. Garnish with fresh rosemary if desired and serve immediately.