Heat the oil in a large frying pan over a medium heat, add the mushrooms and cook, stirring frequently, for 5 minutes. Add the mascarpone and cook over a high heat for 1 minute until thickened. Season with salt and pepper.
Meanwhile, put the pasta sheets in a large roasting tray and cover with boiling water. Leave to stand for 5 minutes, or until tender. Drain off the water.
Lightly oil an ovenproof dish and place 3 pasta sheets over the base, slightly overlapping. Top the pasta with a little of the taleggio, one-third of the mushroom sauce and one-third of the spinach. Repeat the process with 2 more layers then top the final layer of pasta with the remaining taleggio.
Place the lasagne under a preheated hot grill and cook for 5 minutes until the cheese is golden brown. Serve immediately.