Preheat Oven: Preheat the oven to 400ºF (200ºC). Grease a large 9x13 rectangular roasting/lasagna pan and set aside.
Sauté Onion and Garlic: Heat 1 Tbsp olive oil in a large frying pan over medium-high heat. Add onion and sauté until translucent, then add minced garlic and cook for an additional minute until fragrant.
Cook Carrots, Zucchini, and Peas: Add sliced carrots and cook for 4 minutes, followed by zucchini, and cook until both are just softened, adding more oil if necessary. Stir in the peas and cook until heated through.
Add Spinach and Season: Stir in chopped spinach until wilted. Season with salt and pepper. Turn off the heat.
Ricotta Mixture
Prepare Ricotta Mixture: In a bowl, combine ricotta, egg, salt, pepper, and roughly chopped thyme. Stir until well-mixed, then place in the fridge until ready to use.
Bechamel Sauce
Melt Butter: In a new medium-large skillet, melt the butter over low heat.
Add Flour: Once melted, increase heat to medium and whisk in the flour. Cook for 1 minute while whisking.
Add Milk Gradually: Pour the milk in small increments, whisking continuously to ensure the sauce stays smooth. Each time the mixture starts to thicken, add more milk and keep whisking. Continue this process until all the milk has been incorporated.
Cook Until Thickened: Once all the milk is added, continue cooking over medium heat, whisking constantly until the sauce thickens, being careful not to let it boil.
Season: Reduce heat to low, whisk in the nutmeg, and season with salt and pepper to taste.
Add Cheese: Stir in the shredded cheese until melted and the sauce is smooth. Turn off the heat and set aside.
Assemble
First Layer: Place one layer of lasagna sheets (4 sheets) in the bottom of the greased pan.
Spread Ricotta Mixture: Spread half of the ricotta mixture evenly over the lasagna sheets.
Add Vegetables: Spread half of the vegetable mixture evenly over the ricotta layer.
Layer White Sauce: Pour one-third of the white sauce over the vegetable layer, spreading it evenly.
Repeat Layers: Add another layer of lasagna sheets (4 sheets), spread the remaining ricotta mixture, then add the remaining half of the vegetable mixture, followed by the second third of the white sauce.
Final Layer with Béchamel Sauce: Place the final layer of lasagna sheets and spread the remaining one-third of the béchamel sauce evenly on top. Finish by sprinkling the remaining grated cheese (1 cup/100g) over the sauce.
Bake: Bake for 40-45 minutes until the top is golden and cripsy and the filling is bubbling.
Rest: Remove from the oven and let rest uncovered for 5-10 minutes to set. Resting the lasagna will make it thicker and prevent it from being sloppy when served, this step however is optional.
Garnish and Serve: Garnish with fresh basil if desired. Serve and enjoy!
Notes
Store: Keep leftovers in an airtight container in the fridge for 2-3 days.Freeze: Wrap individual portions or the entire lasagna in cling film and foil. Freeze for up to 2-3 months.Reheat: Thaw in the fridge overnight, then reheat in a 350ºF (175ºC) oven, covered with foil to prevent over-browning.Make Ahead: You can assemble the lasagna up to a day in advance. Once assembled, cover the dish tightly with cling film and foil, then refrigerate overnight. When ready to bake, allow the lasagna to sit at room temperature for 30 minutes before baking as per the recipe instructions.