In a large skillet over medium high heat, cook sausage and onion (crumbling sausage with a wooden spoon as it cooks) until sausage is cooked and no longer pink (about 6 to 7 minutes). Drain off fat.
Add garlic and cook until fragrant (while stirring), about 30 seconds.
Stir in pasta sauce, tomato sauce, water, Italian seasoning, salt and pepper.
Add lasagna noodle pieces. Stir to combine and coat noodles with sauce mixture. Bring to a boil. Reduce heat to medium low (or as needed to maintain a simmer), cover and simmer until lasagna noodles are tender, about 15 to 20 minutes.
Stir in the spinach until slightly wilted, then stir in the mozzarella cheese.
Remove from heat and dollop with the ricotta cheese. Cover and let stand 5 minutes.
Sprinkle with grated Parmesan.