1-First, preheat your oven to 350°F (177°C) and grease a 9×13 inch baking pan with oil to get started. This ensures your lasagna doesn’t stick and bakes evenly for that perfect golden finish.

2-Next, heat the olive oil in a skillet over medium heat and cook the onions and mushrooms until the onions are translucent, which takes about 10 minutes. Then, add the garlic, spinach (in batches if needed), and ¼ teaspoon salt, cooking until the spinach wilts and turns bright green. In a bowl, mix the ricotta cheese, eggs (if using), ½ teaspoon salt, and ¼ teaspoon pepper until smooth for a creamy filling.

3-Now, spread 1 cup of pasta sauce evenly over the bottom of the prepared pan and place 3 lasagna sheets lengthwise to cover it. Layer half of the ricotta mixture over the noodles, sprinkle with one-third of the parmesan and mozzarella cheeses, add half of the spinach mixture, and dollop ½ cup sauce on top. Repeat with another layer of 3 noodles, the remaining ricotta mixture, one-third of the cheeses, the rest of the spinach mixture, and another ½ cup sauce.

4-For the final layer, add 3 more noodles, then top with the remaining pasta sauce, parmesan, and mozzarella cheeses. Cover loosely with foil and bake for 30 minutes, then remove the foil and bake for an additional 10-15 minutes until bubbly and browned. If you want a golden cheese top, broil uncovered for 1-2 minutes. Let it rest before serving to make slicing easier, and remember to adapt for dietary needs like using vegan cheese or gluten-free noodles.