Heat a large sauté pan over medium heat, add the olive oil and heat through. Add the garlic and sauté, stirring constantly, for 30 seconds.
Add the spinach, tossing to coat all the leaves in the oil. Sauté the spinach, stirring occasionally, until all the leaves have wilted, about 4 minutes. Remove to a large bowl and return the pan to the heat.
Add the butter and allow to melt. Add the flour, whisking to combine. Let the roux cook until a pale golden brown, about a minute. Add the milk, whisking to incorporate.
Bring the mixture to a simmer, then lower the heat to medium-low, whisking until the sauce thickens, about 2 minutes. Return the spinach to the pan and stir to combine. Season with salt then add the sauce over the bowl of spaghetti squash flesh, stirring to combine, and evenly coat the squash.