Pulse the cashews in a blender until a fine powder forms. You’re looking for a flour-like consistency. Be careful not to overmix, or you’ll end up with cashew butter. Remove the cashew powder and set aside.
To the same blender, add the almond milk, garlic, salt, lemon juice and mustard. Blend on high until smooth.
Pour the liquid into a 6-quart electric pressure cooker. Add the spinach, artichoke hearts, water chestnuts, and chicken (and water if using, see recipe notes below). Stir until well combined.
Secure the lid, select the manual setting, and set it to high pressure for 15 minutes. Your instant pot may take anywhere from 25-35+ minutes to come to pressure before the cook time starts.
When the pressure cooker timer is done, quick release the pressure.
Uncover and remove the chicken to a cutting board. Chop into bite-sized pieces, shred, or leave whole.
To the pressure cooker, add the cashew powder, nutritional yeast, and mayonnaise. If the sauce seems dry, add an extra splash of almond milk. Taste and add additional salt or freshly ground pepper to taste.
Add back the chicken and stir. Serve the chicken, topped with the sauce.