Preheat the oven to 450 degrees F.
With a sharp knife, carefully cut both of the squashes in half, lengthwise. Take an ice cream scoop, or large spoon and scrape out all of the seeds. You can discard the seeds.
Line a sheet tray with aluminum foil. Season the spaghetti squash with olive oil, salt, and pepper. Place cut side down, and roast for 30 to 40 minutes until fully cooked. The squash is done when you can easily slide a fork into it. Remove from the oven and let rest until cool enough to handle.
While the squash is cooking, make the marinara sauce.
In a large deep skillet, add the olive oil and garlic slices. Heat over medium high until the garlic starts to sizzle. Make sure it doesn’t brown.
As soon as the garlic starts to sizzle add the tomatoes. Simmer until sauce thickens, about 45 minutes. Season with salt and pepper.
When squash is cool enough to handle, using a large kitchen fork, scrape the strands of squash from the inside of the skin.
Add the strands of squash to a large bowl and toss with marinara sauce.
Garnish with parmesan cheese and basil.