Bring 2 1/2 cups water to a boil in a pot. Combine 1 1/4 cups water, cashews, cornstarch, soy sauce, brewers' yeast, onion powder, lemon juice, basil, garlic powder, salt, and rosemary in a blender and process until smooth. Pour mixture into the boiling water in the pot.
Use remaining 1/4 cup water to rinse out blender and add to mixture in the pot. Cook gravy until thickened, 5 to 7 minutes.
