Preheat and prep the squash. Heat the oven to 400°F (200°C). Halve the spaghetti squash lengthwise and scoop out the seeds.
Drizzle the cut sides with olive oil and season with salt and pepper.
Roast until tender. Place cut side down on a parchment-lined sheet pan. Roast for 35–45 minutes, or until the flesh easily shreds with a fork. Larger squash may need a few extra minutes.
Crisp the bacon. While the squash roasts, cook the bacon pieces in a large skillet over medium heat until crisp, about 6–8 minutes.
Transfer to a paper towel–lined plate. Reserve 1–2 tablespoons of the bacon fat in the skillet and discard the rest.
Make the garlic butter. Lower the heat to medium-low. Add butter to the skillet with the reserved bacon fat. When melted, stir in the minced garlic and red pepper flakes.
Cook until fragrant, about 30–60 seconds. Do not brown the garlic.
Shred the squash. Remove the squash from the oven. When cool enough to handle, use a fork to pull the strands into a bowl. Season lightly with salt and pepper.
Toss it all together. Add the squash strands to the skillet with the garlic butter.
Toss gently to coat. Stir in most of the crispy bacon, Parmesan, and parsley. Taste and adjust seasoning with more salt, pepper, and a squeeze of lemon if you like brightness.
Finish and serve. Plate the squash and top with the remaining bacon, a sprinkle of Parmesan, and extra parsley.
Serve warm.