Add potato slices and simmer 15 minutes, or until tender.
Drain will.
Spread bottom of hotel pan with 2-1/2 cups Tomato Sauce. Top with 1/3 of the cooked potatoes (approximately 4-1/3 cups). Top with 1/3 Spinach-Ricotta Filling (approximately 3-3/4 cups).
Repeat layering pattern twice.
Spread top with remaining tomato sauce and sprinkle evenly with Mozzarella.
Cover with foil and bake for 1 hour at 400 degrees F.
Remove foil and bake 20 more minutes.
Remove from oven and let sit 15 minutes before serving.
To serve: Cut lasagne in 24 pieces (approximately 8 oz. each). Lift each piece with a spatula onto a dinner plate.
Tomato Sauce
Sauté onions and garlic in olive oil in a stockpot over medium heat, 5 minutes, or until softened.
Coarsely chop tomatoes. Add to onions along with any liquid.
Stir in remaining ingredients. Bring to a boil, reduce heat and simmer 20 minutes.
Spinach-Ricotta Filling
Drain Spinach.
Mix with remaining ingredients until well combined.