Blend the spinach leaves, cashews and 2 cups of water together in a blender until you get a smooth sauce ( a little grainy texture from the cashews are fine, if you want it really smooth you can soak the cashews in water in advance for about 15 minutes). Keep this prepared spinach pesto sauce aside.
Break the spaghetti noodles in half and add the noodles to the instant pot.
Add the 6 cups of water and also add enough salt, garlic powder and onion powder. Let the noodles soak in the water for about 5 minutes. After five minutes, gently stir the noodles. Make sure the noodles are not stuck at the bottom of the pot, or the noodles can burn when you cook.
Place the butter on top.
Close the instant pot with lid in sealing position, press manual and cook in high pressure for 3 minutes.
Once the cooking is done, press the 'keep warm/cancel' button and wait for a natural pressure release for 12 minutes, after which you can do a careful and gradual, controlled manual pressure release.
Once the pressure is all released, carefully open the lid, add the olive oil and gently stir the pasta. If any noodles is clumped together, use a fork to separate the noodles if needed.
Add the prepared pesto sauce to the cooked noodles, stir in to coat the noodles.
Add the cheese, heavy cream and milk (if using), stir in to mix with the cooked pasta. Serve hot for the best taste and if you want it saucy! The noodles will absorb most of the sauce as it cools down, but it still will be delicious!