Prepare according to the manufacturer's instructions. This typically entails boiling a pot of water, cooking for 6-10 minutes, or until barely al dente.The pasta will cook another hour in the oven, so make sure not to overcook them during this stage!
Drain and set aside.It can help to lightly coat the lasagna sheets in oil to keep them from sticking together. Although I usually just lay them over the strainer and make sure that they're not touching too much.
Cheese Sauce
Grate cheese into a small bowl and toss with cornstarch. Make sure that all of the pieces are lightly coated.This will help the cheese from separating when you cook it. If you're having a tedious time getting the cornstarch coated without clumping, add the mixture to a large Ziplock bag and shake.Make sure to reserve about 1 cup of Manchego. This will go on top of the final layer.
Meanwhile, add wine and lemon juice to the saucepan and bring to a simmer.
Once the wine mixture begins to simmer, slowly stir in the cheese about 1/4 cup at a time. Make sure to stir constantly until fully incorporated.The key to a creamy fondue is adding in the cheese gradually. If it’s added too quickly, the mixture will separate!
Once all of the cheese is incorporated and the mixture is smooth, stir in the Kirsch, mustard, garlic, salt, and pepper.
Lasagna
Preheat the oven to 350 degrees Fahrenheit.
Lightly coat the bottom and sides of a large baking baking dish with olive oil and set aside.Mine is 13x9 inches, but any larger-sized baking dish will do. You can always cut the lasagna sheets to different sizes.
Arrange noodles in the bottom of the baking dish. They should slightly overlap to keep the noodles from separating.
Add 1/3 of the spinach to the lasagna noodles. Followed by a layer of tomatoes and fresh mozzarella. Then, drizzle with 1/3 of the cheese sauce.
Repeat these layers until you have all 3 layers and top off with the reserve amount of Manchego.
Bake for 50-60 minutes. The lasagna should be thoroughly heated throughout and the cheese on top will begin to brown. Let cool slightly and cut into 2-inch squares. Serve immediately.