Preheat oven to 375F.
Bring a large pot of salted water to a boil; cook the spaghetti until al dente. Drain and set aside.
In a large, deep skillet, add olive oil to the pan and heat oven medium heat. Add the chopped bacon and mushrooms to the pan and cook until the fat has rendered, the bacon is crisp, and the mushrooms are golden. Remove with a slotted spoon and set aside.
Add butter to the pan and whisk in the flour for 1-2 minutes. Slowly add the chicken broth and milk; simmer until beginning to thicken. Season with garlic salt and pepper. Stir in 2 1/2 cups of the shredded cheese, a handful at a time.
Add in the chicken, broccoli, and cooked spaghetti, reserved mushrooms and bacon. Toss until combined.
Spray a 13x9 baking dish with cooking spray. Pour the pasta mixture into the pan and top with remaining cheese.
Transfer to preheated oven for 15 minutes, until cheese is melted.