Add the ⅛ cup of olive oil to a large frying pan and sauté the onions, carrots and celery over medium-high heat, for about 15-20 minutes. Cook lentils according to packaged directions and set aside.
Cook until the vegetables are tender, lightly browned and most of the moisture from the mushrooms has come off.
Add the tomato paste, stirring constantly, and cook for another 3 minutes. Set aside to cool.
Add oats, walnuts and garlic to food processor.
Pulse oats, walnuts and garlic in a food processor until they are bread crumb consistency.
Add in cooked veggies and pulse until well incorporated (see Helpful Tips about adding lentils at this time). Remove mixture and put into a bowl.
Add lentils and seasonings.
Mix together well. Scoop out meatballs using an ice cream scoop to a parchment lined cookie sheet. Bake at 400F for 25-30 minutes - monitor the meatballs during the last few minutes of cooking so they do not burn.