I don't remember the last time I made a traditional meat & tomato sauce lasagne. Not that I don't like it, but I love this variation with spinach & carrots even more. I usually make this in a 9" x 9" casserole (more on that below), which means you have to trim the noodles to fit the pan and the layers are quite thick. Alternately you could use a 9" x 13" pan, which would result in thinner layers. Depending on how much crunch you like, you may want to parboil the carrots for a minute or two. I often add them to the spinach towards the end of it's cooking time, and then you can just drain all the veggies together before adding them to the rest of the filling.
Miscellaneous question: why do lasagne noodles always shred when you boil them??9 cooked lasagne noodles
2+1/2 cups ricotta
1 cup shredded mozzarella and/or fontina cheese (I used an Italian cheese blend this time)
1 cup diced carrot
1 package frozen spinach, cooked & well-drained
1 egg, slightly beaten
1 large onion, finely diced (use 1/2 for filling, 1/2 for sauce)
Mix all ingredients and season with salt & pepper. Spread one layer of noodles in a 9" x 9" or 9" x 13" pan and top with half the filling. Top with layer of noodles, other half of filling, and final layer of noodles. Go to sauce directions...
1 - 2 tablespoons olive oil
2 tablespoons flour
1 cup chicken broth
1/2 cup ricotta
Sauté remaining onions in the oil until soft. Add flour and cook for a minute, then whisk in broth and remaining ricotta. Bring to a boil and simmer until it thickens slightly (shouldn't take long). Season with salt & pepper, if necessary. Pour sauce over the lasagne layers. Sprinkle with additional mozzarella if desired. Cook at 375° for 30-45 minutes, until bubbly around the edges and top is lightly browned.
Below is a favorite casserole that my mother made. Unfortunately, after years of service it has developed this scary crack down the middle. Every time I use it I'm afraid it may be the last. But it held up this time!