Step 1Heat a large, heavy-based saucepan with oil spray over a medium-low heat. Add onion, carrot and celery. Cook, stirring occasionally, for 6-8 minutes or until vegetables soften. Add garlic and thyme and cook, stirring, for 1 minute. Add tomato paste and cook, stirring, for 2 minutes. Add tomatoes and lentils and cook, stirring occasionally, for 10 minutes or until sauce thickens. Remove from heat and set aside.
Step 2Spray a large pan with oil. Add mushrooms and cook, stirring, for 3-4 minutes or until slightly soft. Season and remove from heat.
Step 3Preheat oven to 180°C. Lightly oil a 2L ovenproof dish. Combine ricotta, egg, milk and nutmeg in a bowl.
Step 4To assemble lasagne, place 1 lasagne sheet in the base of the prepared dish. Top with one-third of the lentil mixture, scatter over one-third of the mushrooms and smooth over one-third of the ricotta mixture. Repeat these steps. Top with one-third lasagne sheet, the remaining lentils and mushrooms then a fourth lasagne sheet and finally the remaining ricotta. Sprinkle over parmesan.
Step 5Bake for 30 minutes or until top is golden and bubbling. Set aside to rest for 5 minutes before cutting.