HEAT oil in a large frying pan over medium-high. Add onion and cook until softened, 3 min. Add mushrooms and cook until tender, 4 to 5 min. Crumble in sausage and cook, breaking up with a wooden spoon, until no pink remains, 2 min. Stir in passata, then remove from heat.
STIR egg, ricotta, spinach, 1 cup of the mozzarella, parmesan, garlic powder and salt in a medium bowl until combined.
SPRAY inside of a 5-qt. slow cooker insert with oil. Cover bottom with 1 cup sausage mixture. Top with a lasagna sheet, cutting to ﬁt (piece together more than 1 sheet if needed to create 1 layer). Spread 1 1/3 cups of ricotta mixture over lasagna. Repeat layering twice more, ending with 1 cup sausage mixture. Sprinkle with remaining mozzarella. Cover and cook for 5 hours on low. Open lid and let stand for 10 min before serving.