Place squash cut-side down in a microwave-safe dish; add 2 tablespoons water. Microwave, uncovered, on High until tender, 10 to 15 minutes. (Alternatively, place squash halves cut-side down on a rimmed baking sheet. Bake at 400 degrees F until tender, 40 to 50 minutes.)

Meanwhile, combine spinach and the remaining 1 tablespoon water in a large skillet over medium heat. Cook, stirring occasionally, until wilted, 3 to 5 minutes. Drain and transfer to a large bowl.

Position rack in upper third of oven; preheat broiler.

Use a fork to scrape the squash from the shells into the bowl. Place the shells on a baking sheet. Stir artichoke hearts, cream cheese, 1/4 cup Parmesan, salt and pepper into the squash mixture. Divide it between the squash shells and top with the remaining 1/4 cup Parmesan. Broil until the cheese is golden brown, about 3 minutes. Sprinkle with crushed red pepper and basil, if desired.