Combine spinach, ricotta cheese, egg yolks, garlic, cheddar cheese, nutmeg, 1/2 the herbs, mushrooms, onion, pepper and salt in a large bowl.
Fill dry cannelloni tubes with spinach mixture. Place tubes into baking dish in a single layer. Pour tomato paste over filled tubes ensuring you cover all the pasta. Drizzle over olive oil, sprinkle with freshly ground black pepper and remaining herbs, and sprinkle the reserved cheese.
Cover, either with baking lid or aluminium foil, and bake in preheated oven for approximately 20 minutes. Uncover and bake for a further 20 minutes, or until top is crisp and pasta is tender when tested with a skewer. Serve with garlic bread and a fresh green salad.