Use a blender to blitz the rolled oats into fine pieces. If you are using leftover lentil sauce from the night before (from my spaghetti bolognese recipe), stir into the mixture. If making from scratch, set aside and proceed to step 2 to make the base
Dice the shitake mushrooms into fine pieces. Use a food processor if you have one
Heat a medium size pot and add the garlic, oil (or water), and diced mushrooms. Stir for 3 to 4 minutes, or until mushrooms are brown
Add tomato paste, oregano and fresh basil. Stirring for another 2 minutes
Add lentils, canned tomatoes and vinegar. Bring the pot to simmer and then reduce heat. Allow the sauce to simmer on very low heat for approximately 10 minutes
Turn of heat, and transfer to a deep baking dish
Stir in the oats
Spread tomato sauce evenly across the top