In a large saucepan over high-medium heat, bring 2 gallons of water to a boil.
In the meantime, add 2 tablespoons of olive oil to large pan over medium heat.
Add the chopped onion, cook for 2-3 minutes until tender, stirring occasionally and add the minced garlic, cook for another 2 minutes.
Add the chopped bell pepper and mushrooms stir and cook for another 4-5 minutes. Add the tomato paste and ground chicken. Season with salt and pepper. Mix well to combine, and cook for about 5 minutes.
Once the chicken is almost cooked add the spinach and cook until the spinach has shrunk in size and is tender about 5 minutes. Turn off the heat.
In the meantime, the water must be boiling, add some salt and 2 tablespoons of oil and add the lasagna sheets. Cook based on box instructions, but usually, it takes 7-9 minutes for the sheets to cook. Transfer lasagna sheets to a colander, give them a bath in cold water and drain.
Start assembling the lasagna. I had to trim the length of the lasagna sheets to fit my 9inch pan. I used 5 sheets for the bottom layer, 5 for the middle layer and 6 for the upper layer.
Preheat oven to 400F.
Start by greasing the lasagna pan with the remaining 1 tablespoon of oil.
Lay 5 lasagna sheets on the bottom of the pan, top it with half of the chicken mixture, followed by 5 ounces of marinara sauce, and followed by 1½ cups of grated mozzarella cheese. Repeat this for the second layer of lasagna sheets -> chicken mixture -> marinara sauce -> cheese. Top with the final layer of 6 lasagna sheets (I also added on top the pieces from the trimmed sheets), cover with the remaining 5 ounces of marinara sauce and top with the remaining 1 cup of cheese.
You can place lasagna in the fridge for 24 hours, or you can bake it immediately for 30 - 40 minutes.
Let lasagna cool down 15-20 minutes before serving, as this will facilitate cutting it.
You can turn on the broil for the last 3-4 minutes of baking for a golden brown top, but watch it closely not to burn it.
Garnish with fresh herbs and serve. Enjoy!