Place the lentils, millet, black rice and oats in a fine mesh strainer and rinse well with water. Tap the strainer against the sink to remove as much excess water as possible.
Transfer the mixture to the Instant Pot (IP), along with the (new) water. Lock the lid into place, making sure the nozzle is in the sealing position.
Use the Manual (or Pressure Cooking) mode and set the timer for 15 minutes. Use the natural release method when the timer goes off.
Remove the lid and fluff the grains with a fork and add the vanilla. Cool to room temperature, then promptly store in your fridge.
Once you have your grains made, heat up however much you want in a saucepan with some non dairy milk. Place in a bowl and top with chopped apple, chopped walnuts and a bit of maple syrup. Add a bit more non dairy milk if you like.