Cut the spaghetti squash in half lengthwise and scoop out the seeds using a spoon.
Add 1 cup of water to the Instant Pot and place the metal trivet inside.
Put the spaghetti squash halves cut-side-up on the trivet.
Secure the lid on the Instant Pot, set the valve to the sealing position, and cook on manual high pressure for 12 minutes.
After cooking, allow a natural pressure release for 5 minutes, then perform a quick release for any remaining pressure.
In a food processor or blender, combine the basil leaves, toasted walnuts, garlic, nutritional yeast, and a pinch of salt and pepper.
While the processor is running, slowly stream in the olive oil until well combined. Taste and adjust seasoning as needed.
Once the pressure has been released, carefully open the lid and remove the spaghetti squash.
Use a fork to scrape the spaghetti squash flesh into strands.
Place the spaghetti squash strands in a mixing bowl, and add the prepared pesto. Toss until well coated.
Divide the spaghetti squash with pesto among bowls, and garnish with halved cherry tomatoes if desired.