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1
Combine the lentils and 2 quarts water in a medium stockpot and bring to a boil over high heat. Reduce the heat to low and simmer until the lentils are soft (but not falling apart), about 25 minutes. Drain the lentils and allow to cool.
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2
Add 1/4 cup of the olive oil to a large frying pan and saute the onions, carrots, celery, garlic, thyme, and salt over medium-high heat, stirring frequently, for about 10 minutes, until the vegetables are tender and just beginning to brown. Add the tomato paste and continue to cook, stirring constantly, for 3 minutes. Add the mushrooms and cook, stirring frequently, for 15 more minutes, or until all the liquid is absorbed. Transfer the mixture to a large bowl and allow to cool to room temperature. When cool, add the lentils to the vegetable mixture.
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3
Add the eggs, Parmesan, bread crumbs, parsley, and walnuts to the cooled vegetable mixture and mix by hand until thoroughly incorporated. Place in the refrigerator for 25 minutes.
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4
Preheat the oven to 400u00b0F.
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5
Drizzle the remaining 2 tablespoons olive oil into a 9x13-inch baking dish and use your hand to evenly coat the entire surface. Set aside.
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6
Roll the mixture into round, golf ball-size meatballs (about 1 1/2 inches), making sure to pack the vegetable mixture firmly. Place the balls in the prepared baking dish, allowing 1/4-inch of space between the balls and place them in even rows vertically and horizontally to form a grid.
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7
Roast for 30 minutes, or until the meatballs are firm and cooked through.
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8
Allow the meatballs to cool for 5 minutes in the baking dish before serving.
