Heat oil in a large skillet over medium heat. Add onion; cook 10 minutes or until onion is browned, stirring occasionally. Stir in garlic and spinach. Reduce heat, cover, and cook 3 minutes or until spinach is tender. Set aside.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, milk, salt, black pepper, and red pepper in a small saucepan, stirring with a whisk. Bring to a boil over medium-high heat, stirring frequently. Reduce heat and simmer 1 minute, stirring frequently. Add 2 cups milk mixture to spinach mixture. Cover remaining milk mixture, and set aside.
Spread 1/2 cup marinara sauce in bottom of a 13 x 9-inch baking dish coated with cooking spray. Arrange 3 lasagna noodles over sauce; top with half of spinach mixture. Top with 3 lasagna noodles, 1 cup marinara sauce, and 3/4 cup cheese. Layer 3 more lasagna noodles, remaining spinach mixture, and remaining 3 lasagna noodles. Top with remaining marinara sauce. Pour reserved milk mixture over the top, and sprinkle with remaining 3/4 cup cheese. Bake at 375° for 50 minutes or until lasagna is browned on top. Garnish with parsley sprigs, if desired.
If you make this lasagna as written you'll end up with one of the blandest lasagnas ever. I doubled the seasoning in the milk mixture and added copious amounts to the spinach. I ended up with a decent result.
The only changes I made were to the milk mixture. I added a little more salt and pepper, fresh parsley, three times more garlic, and about a half cup of Parmesan cheese (was more like an Alfredo sauce). The rest I left as is and it was perfect!!! With these changes, it was the best lasagna I ever had!! Five stars with the minor changes.
This came out of the oven smelling amazing and looking perfect. Unfortunately, the flavor was off for me. I don't know if it was that the milk mixture was bland or there were too many onions. Aesthetically, it was the best lasagna I've ever made but definitely not from a taste standpoint. I'd pass on making this again.
3 stars = good solid recipe. After reading the reviews, I felt it was important to taste the spinach mixture before adding it to the lasagna. I highly recommend this. I ended up adding some salt and pepper while it was cooking, and then adjusting after I added the milk mixture. I also found that my bottle of marinara didn't quite have the 'bite' that I wanted to go with the milder flavor of the spinach mixture, so I added some red wine vinegar. I might add some red wine vinegar to the spinach mixture next time. I also added extra garlic per previous comments recommendations, and cut the red pepper in half, but since we don't have little people in our house, I would probably go ahead with the full amount next time I make it. I worried that the noodle to filling to ratio was going to be too heavy with noodles, but in eating it, I found it balanced. I cooked the whole wheat noodles per the package directions, and found them to be over done, so will under cook them next time.