Halve the squash lengthwise and scoop out the seeds.
Sprinkle the cut sides with 1/2 teaspoon salt, then brush both sides with olive oil.
Put the squash, cut-side up, in a baking dish and cover tightly with aluminum foil.
Roast 20 minutes, then uncover and continue roasting until the squash is tender, about 35 more minutes.
Make the sauce: Heat 3 tablespoons olive oil in a large pot over medium-high heat.
Add the remaining vegetable paste and cook, stirring occasionally, until it looks dry, about 5 minutes. Stir in the tomato puree; rinse each can with 1 cup water and add to the pot.
Stir in the basil, oregano and 1 1/2 teaspoons salt.
Bring to a simmer, until the sauce thickens and then remove the basil.
Use a fork to scrape the spaghetti squash flesh into strands; transfer to a large bowl and toss with 2 tablespoons grated parmesan.
Season with salt.
Divide the squash among bowls and top with sauce and the remaining 1 tablespoon parmesan.
