Step 1: Prepare the Lasagna Sheets

1. Boil a large pot of water and cook the lasagna sheets according to the package instructions until al dente.
2. Drain and toss them with a little olive oil to prevent sticking. Set aside.

Step 2: Cook the Chicken

1. In a bowl, coat the chicken tenders with salt, pepper, cayenne, garlic powder, Italian seasoning, and olive oil. Massage the spices into the chicken for even coating.
2. Heat a cast-iron skillet or non-stick pan over medium-high heat.
3. Cook the chicken for about 4-6 minutes per side, or until golden brown and fully cooked. Be careful not to overcook, as the chicken will continue to cook slightly in the oven.
4. Transfer the chicken to a cutting board, let it rest for 5 minutes, then chop it into small bite-sized pieces. Toss the chopped chicken in its own juices for maximum flavor. Set aside.

Step 3: Make the Spinach Alfredo Sauce

1. In a large pan, melt the butter over medium heat.
2. Add the finely chopped onion and sauté until soft and translucent (about 3-5 minutes).
3. Add the minced garlic and sauté for another 1-2 minutes until fragrant.
4. Stir in the salt, pepper, chili flakes, and Italian seasoning. Cook for 1 minute.
5. Sprinkle in the flour and cook for 1-2 minutes, stirring constantly, to eliminate the raw flour taste.
6. Slowly pour in the milk and heavy cream, stirring continuously to prevent lumps.
7. Reduce the heat to low and let the sauce simmer, stirring occasionally, until it thickens slightly.
8. Remove from heat and stir in the grated Parmesan cheese and chopped spinach. Mix well and let the spinach wilt in the warm sauce.
9. Taste and adjust seasoning as needed.

Step 4: Assemble the Lasagna

1. Preheat your oven to 375°F (190°C).
2. Lightly grease a 9×13-inch baking dish with butter or oil.
3. Spread 2-3 tablespoons of the Alfredo sauce evenly on the bottom of the dish.
4. Place 3 lasagna sheets in a single layer (trim if needed to fit).
5. Spread a generous layer of sauce over the noodles, followed by ⅓ of the chopped chicken and a sprinkle of ¾ cup mozzarella cheese.
6. Repeat the layering process for two more layers (lasagna sheets, sauce, chicken, and cheese).
7. For the final layer, place the remaining lasagna sheets, spread the remaining Alfredo sauce on top, and sprinkle with 1 cup of mozzarella cheese.

Step 5: Bake the Lasagna

1. Insert 6-8 toothpicks into the lasagna to prevent foil from sticking to the cheese.
2. Cover the baking dish with foil and bake for 40 minutes.
3. Remove the foil and broil for 2-3 minutes until the cheese is golden brown and bubbly.
4. Remove from the oven and let the lasagna rest for 15 minutes before slicing.