Heat oil in a large saucepan over medium heat. Add onion and garlic. Cook, stirring for 5 minutes until onion softens. Add tomato paste and cook for 1 minute.
Add mince and increase heat to medium-high and cook for 3-4 minutes until browned. Add mushrooms and thyme and cook for a further 3 minutes.
Add chopped tomatoes. Bring to the boil and simmer on low for 30-35 minutes until sauce thickens. Season to taste.
In a bowl, combine ricotta, egg, parmesan and remaining tasty cheese. Stir through spinach and season to taste.
Preheat oven to 200°C/180°C fan-forced. Lightly grease a 3 litre ovenproof dish.
Spread ½ cup sauce over base of dish. Top with 1 lasagne sheet, trimming to fit. Top with one third of the sauce, ¼ cup tasty cheese and lasagna sheet. Continue layering with remaining sauce, cheese and lasagne sheets.
Blanch spinach in a small pan of boiling water until just wilted. Drain and squeeze out excess moisture. Chop finely.
Spread mixture over lasagne. Bake uncovered for 40-45 minutes until golden. Stand 5 minutes before serving with salad.