How to Make It
Pour boiling water over porcini in a small bowl; let stand 15 minutes. Remove porcini with a slotted spoon; finely chop porcini, and reserve soaking liquid.
Heat a large skillet over medium heat. Add oil to pan; swirl to coat. Add onion, carrot, and celery; cook 5 minutes, stirring occasionally. Add cremini mushrooms and garlic; cook 6 minutes or until mushroom liquid evaporates, stirring occasionally. Add beef; cook 3 minutes, stirring frequently. Add tomato paste; cook 1 minute, stirring constantly. Stir in porcini, oregano, salt, pepper, and tomato sauce; pour in porcini liquid, stopping before grit at bottom of bowl reaches sauce. Bring to a boil; reduce heat, and simmer, uncovered, 12 minutes or until thickened, stirring occasionally to keep sauce from sticking.
Run squash through a spiralizer to create noodles, or cut into long noodles with a julienne peeler; set aside. Cook pasta according to package directions, omitting salt and fat, and adding butternut noodles during last 2 minutes of cooking time. Drain.
Stir walnuts into sauce; spoon over pasta mixture. Sprinkle with cheese.