Heat oven to 400F. Slice one end of the spaghetti squash off. Slice in half lengthwise and scoop out the seeds. Repeat for next squash.
Place squash on a parchment-lined rimmed baking sheet, drizzle cut sides with olive oil, and season with salt and pepper. Place cut side down and roast 40-45 minutes, or until easily pierced with a fork.
While squash roasts, combine ricotta cheese, egg, Italian seasoning blend to a bowl. Mix well.
Using a fork, gently scrape out the squash strands and transfer to the bowl with ricotta mixture. Stir in 1 cup sauce, and mix until mostly combined but still streaky.
Return the mixture to the hollowed-out squash boats. Dollop additional sauce over the squash and scatter with fresh mozzarella. Top with remaining parmesan.
Bake until cheese is melted and browned in spots, 20-25 minutes. Garnish with basil, if desired, and dig in!