Preheat oven to 375°. In a medium bowl, combine egg, ricotta cheese, 1 cup mozzarella cheese, salt & pepper. Set aside.
Lightly oil a 13x9x2 inch baking pan. Arrange 4 noodles in the bottom of the pan. Top with spinach. Spoon ½ of the ricotta cheese mixture over the spinach and spread evenly. Spoon 1/3 of the pesto over the ricotta mixture & spread evenly. Add 4 more noodles, then top with the mushrooms. Spread the remaining ricotta cheese mixture over the mushrooms and half the remaining pesto over that. Top with the remaining 4 noodles and the remaining pesto. Slice the tomatoes & place on top of lasagna.
Bake, covered, for 40 minutes. Uncover & sprinkle with remaining mozzarella cheese. Bake, uncovered, for 5 minutes. Let stand for 15 minutes before serving.
Lemon-Parsley Pesto: In a food processor, combine 4 cups fresh spinach (8 oz.), ½ cup flat-leaf parsley, 1/3 cup freshly grated parmesan cheese, 1/3 cup toasted pine nuts or walnuts, 2 crushed garlic cloves, 2 tbsp olive oil, 2 tsp. lemon zest, 2 tbsp lemon juice & ¼ tsp salt.
Puree until nearly smooth. (If using a blender, add 2-4 tbsp water.)
Serves 8