Wilt the spinach in a hot frying pan and drain on paper towels, pressing out most of the water.
Brown the ground beef in a large skillet and drain off any excess fat. Pour the jarred marinara sauce into the skillet with the ground beef and simmer for 10-12 minutes.
Boil the lasagna noodles in lightly salted water per package directions. Drain in a colander.
Grate the hard mozzarella using the largest setting on a box grater. Slice the fresh mozzarella into thin pieces.
Pour a small amount of sauce in the bottom of a 9x13 baking dish or (2) 8x8 inch baking dishes. Spread it out to cover the entire bottom thinly.
Start assembling your lasagna with a layer of noodles followed by the cheeses and spinach. Ladle on marinara sauce. Continue layering, ending up with a layer of noodles followed by a light layer of sauce and grated Parmesan on top.
Bake in a 350 degree oven for 45 minutes or until it is bubbly and hot. For the first half hour of cooking lay a sheet of aluminum foil over the top. Do not press it down, just let it lie on top to keep the lasagna from drying out. Remove foil for the final 15 minutes of cooking.
Take the lasagna out of the oven and allow it to rest for 10 minutes.(Very important!) Any extra liquid will be reabsorbed and your lasagna will not be soupy. Cut into squares for serving.