Prep: Preheat oven to 400°F (204°C). Cut spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the insides with most of the olive oil and rub to evenly coat. Place cut-side down on a baking sheet and prick the outside with a fork. Spread broccoli florets onto the same baking sheet and drizzle with remaining olive oil, tossing to coat it evenly.
Roast: Roast for about 30 minutes, or until squash is fork tender but still a little firm. Broccoli should be lightly browned (if it cooks more quickly, take it out of the oven earlier than the squash). When squash is finished, flip it over and allow to cool enough to touch.
Spaghettify: Use a fork to scrape strands from the inside of the spaghetti squash.
Assemble: In a large bowl stir together spaghetti squash strands, roasted broccoli, Alfredo sauce, and ½ cup of the cheese. Pour into a large casserole dish and gently pack so that it's in an even layer. Sprinkle with remaining ½ cup of cheese.
Broil: Switch the oven to broil and add casserole dish. Cook for just a few minutes, watching closely until the cheese has melted and turned golden brown.
Store leftovers in an airtight container in the fridge for 4 to 5 days.