I found this recipe in a Rachel Ray magazine then tweaked it to be my own. It is absolutely fantastic. I love it so much. My family loved it too. It is a little bit more time consuming that some other recipes but well worth the raves. I think you could possibly use already cooked lasagna noodles and I may try that next time to see how they turn out. However, this recipe is so good, I hate to tamper with it too much. It is a company dish and a dish that you would get in a restaurant. Lovely plated and served up for your family and tastes simply delish as Rachel would say. Try it, you will agree.
In a medium saucepan, melt 3 tablespoons of butter over low heat. Stir in the flour and cook for two minutes. Slowly whisk in the hot milk, then the smashed garlic cloves (I added two instead of one) and 3/4 tsp. salt. I also found this mixture not very salty and had to add more so taste this mixture when it has thickened. Cook and bring garlic sauce to boil, whisking continually for 3-5 minutes until thickened. Set aside.
In a large pot of boiling water that is well salted and a little oil so the noodles will not stick, cook the lasagna noodles for 8 minutes until they are not quite ready to eat but hard to bite. Drain and transfer to bowl of cold water to stop cooking process.
Position a rack in the middle of the oven and preheat to 375 degrees. In a large skillet, melt the remaining 2 tablespoons of butter over low heat. Add onions and garlic and cook over low heat until the onions and garlic are translucent and softened. Also this may need to have a little salt. This whole dish seemed to be not salty enough so I kept having to salt each portion and I am not a person who adds extra salt. At this point, stir in the mushrooms and increase heat to medium high and cook until mushrooms are dry about 5 - 7 minutes.
Using a food processsor, puree the basil leaves with the garlic cream. Set aside 1/2 cup of the cream. Stir in the remaining basil garlic cream and 1/2 cup of parmesan into the mushroom mixture. Season with salt and pepper.
Grease or spray a rectangular baking pan and spread 3/4 cup of the garlic sauce on the bottom of the pan. Drain the lasagna noodles. Lay noodles on top and alternate your mixture with noodles then top with your reserved sauce and the rest of your parmesan cheese. I used extra parmesan cheese throughout the layers because I had extra cheese and it made it fantastic. I'm sure it is great either way. Be careful not to overuse your filling or you will run out before you use it on all of your noodles. You have nine noodles or three layers of three to use. That is two layers of filling and then the last reserved cream sauce on top. (Personal note: I barely had enough. It almost needs a little more cream sauce and next time I make it, I'm going to figure in a little more filling. I'm not sure how I'm going to do it but I am. I think it needs a little more. Either that or leave a few noodles out but I think the noodles need to be there. I'll probably do a 1/4 of a recipe more of the cream sauce but no more portabellos or onions.)
Bake until golden 40-45 minutes. Let stand for 15 minutes.