Preheat the oven to 350°F.
Brush each half of the squash with 1 teaspoon of the oil and season generously with salt and black pepper. Place the squash, cut sides down, on a baking sheet and roast for 30 to 40 minutes. The squash is cooked when a knife easily pierces through the skin and flesh. Remove from oven and let cool enough to handle, shred the flesh with a fork into spaghetti-like threads and set aside.
Turn the oven up to 400°F.
Spread the pine nuts on a small baking sheet and place them in the oven to toast until golden brown, 3 to 5 minutes, checking often to prevent browning.
Heat the remaining 2 teaspoons of oil in a large sauté pan over medium heat. When the oil shimmers, add the garlic to the pan and cook, stirring constantly, for 2 to 3 minutes.
Add the tomatoes to the pan, season to taste with salt and black pepper and cook until the tomatoes begin to burst, 5 to 6 minutes.
Add the spinach to the pan and season to taste with salt and black pepper. Cook, stirring until the spinach wilts, 3 to 4 minutes.
Divide the spaghetti squash among 4 plates and top with the sautéed spinach and tomatoes. Sprinkle on the pine nuts and fresh basil. Serve immediately. Any leftovers can be covered in an air-tight container and refrigerated for up to 4 days.