How to Make It
Preheat oven to 375°F. Line a baking sheet with aluminum foil; coat with cooking spray. Cut ends off spaghetti squash; discard. Slice squash into 1 1/2-inch rings; scoop out and discard seeds and membranes. Cut lemon in half lengthwise. Squeeze juice from 1 lemon half to equal 1 tablespoon, and set aside. Cut remaining lemon half into 4 wedges. Arrange lemon wedges, squash rings, and shallot quarters on prepared baking sheet. Coat with cooking spray.
Bake until squash is just tender, 40 to 45 minutes. Remove from oven, and set aside until cool enough to handle, about 5 minutes. Remove squash rings from baking sheet; set aside lemon wedges and shallot quarters until ready to serve. Scoop out spaghetti-like strands from inside of squash rings into a bowl using a spoon. (Discard remaining squash.) Stir 1/4 teaspoon of the salt into squash strands in bowl. Cover to keep warm, and set aside.
Fill a bowl with ice water; place a colander inside bowl on top of ice. Fill a medium saucepan with water; bring to a boil over high. Add spinach, parsley, and 1 cup of the basil to saucepan; cook 30 seconds. Remove with a slotted spoon; place in colander submerged in ice water, and let stand 2 minutes. Drain and squeeze out excess water.
Place cooked spinach mixture, oil, garlic, reserved lemon juice, 3 tablespoons of the pepitas, and remaining 1/4 teaspoon salt in a food processor; process until smooth, about 45 seconds.
Add spinach pesto to squash strands; toss gently. Divide squash mixture, lemon wedges, and shallot quarters evenly among 2 shallow bowls. Sprinkle bowls evenly with remaining 1/4 cup basil leaves and 1 tablespoon pepitas.