In a large non-stick frying pan, heat 2 tablespoons olive oil over medium high heat and saute onions, carrots, celery, garlic, thyme and salt, about 10 minutes, until veggies are soft. Add the tomato paste and cook and stir for an additional 2-3 minutes. Tomato paste will darken and thicken a bit.
Add the mushrooms and saute for about 15 minutes until mushrooms have softened and released their moisture. All the liquid should be evaporated and absorbed.
Transfer mixture to a large bowl and allow to cool to room temperature. Once cool, place vegetables and fava beans into a food processor and pulse 2-3 times until slightly broken up. (Seriously - this is a quick pulse, don't over do it!) Return mixture to the large bowl and add the lentils, eggs, Parmesan, bread crumbs, parsley, and walnuts. Mix by hand until thoroughly incorporated. Refrigerate to chill for 25 minutes.
Preheat the oven to 400°F.
Drizzle the remaining olive oil into a 9×13-inch baking dish and use your hand to evenly coat the entire surface.(You can also use cooking spray here). Set aside.
Roll the mixture into round, golf ball-size meatballs (about 1 ½ inches), Pack firmly. Place the balls in the prepared baking dish, allowing ¼-inch of space between the balls. They should all fit on one large cookie sheet.
Roast for 30 minutes, or until the meatballs are firm and cooked through.
Allow the meatballs to cool for 5 minutes in the baking dish before serving. Or cook completely and store in air-tight container and freeze for up to 1 month.