Step 1:
Place the trivet inside the Instant Pot and add the water. Season the halved spaghetti squash with kosher salt and freshly ground black pepper. Set the squash halves on the trivet, secure the lid, and choose the Pressure Cook setting on High for 7 minutes. Follow the manufacturer’s instructions for a quick release of pressure, then remove the lid. Take out the squash and allow it to cool until it can be handled comfortably.
Step 2:
Preheat your oven to the broil setting. Using a fork, carefully scrape out the strands of the spaghetti squash into a large mixing bowl, setting the squash shells aside. Combine the strands with the baby spinach, ricotta, dried oregano, and crushed red pepper flakes. Mix thoroughly. Spoon the mixture back into the reserved squash shells and layer the top with marinara sauce, fresh mozzarella slices, and grated Parmesan.
Step 3:
Arrange the filled squash halves on a large baking sheet. Place them under the broiler until the cheese has melted and turned bubbly, about 5 minutes. Remove from the oven and garnish with freshly chopped parsley before serving.