Bring a pan of water to a rolling boil. Place the spinach and parsley and wilt for 2-4 minutes, or until soft and wilted. Using a slotted spoon or colander, drain the spinach and parsley.
Place in a bowl large enough to fit your immersion blender, Let cool for a few minutes, then blend, slowly drizzling olive oil into the mixture until it becomes silky smooth. Don't add too much otherwise you will dilute the flavour too much.
Slowly add the ice cubes, one by one, until fully blended. If you feel that your blender may brake, rather than fully immersing the blender, slowly 'shave' layers of ice with the blender off the top, placing the blender gently on the ice cube, allowing it to blend a small section, lifting the blender out, then blending another 'layer' again. By this point, the sauce should be room temperature, or warm-ish, and smooth and creamy.
Stir in any flavours you'd like too - mixed herbs, garlic, chilli, lemon or lime zest, lemongrass, Kaffir lime - think fresh and vibrant flavours. Be creative!