Preheat the oven to 375 degrees F. Oil a casserole dish deep enough to hold the lasagna. Place the porcini mushrooms in a bowl with hot water to rehydrate, and allow to soak for 15 minutes.
Cook the lasagna sheets, as per the packaging instructions, in unsalted water.
Drain and rinse with cold water.
Chop the brisket into bite-sized chunks and combine in a bowl with the tomato passata and tomato paste.
Once the porcini mushrooms have soaked for 15 minutes, drain and chop. Also, chop the fresh mushrooms, garlic, and thyme.
Heat butter in a pan and saute the mushrooms and porcini mushrooms with the garlic and thyme.
Melt butter in a saucepan and season with salt and pepper. Once the butter has melted, whisk in flour. Cook gently for 2-3 minutes, gradually whisk in hot milk, stirring constantly. Once it comes to a simmer, add the parmesan cheese, stir until the cheese has melted, and then transfer to a bowl
Spread a ¼ cup of brisket mixture on the bottom of the baking dish. Place 3 of the cooked lasagna sheets on top of the sauce.
Layer with ⅓ of the cheese mixture, then ¼ of the brisket, mushrooms, and spinach. Repeat the process three times. Top with the grated cheese and cheese mixture.
Place the ovenproof dish on a tray to catch any drips. Bake for 45 minutes or until the top is golden brown and crisp around the edges. Serve immediately