In a large skillet melt 4 tablespoons of the butter over low heat. Add the sliced onions, sugar and season with salt and pepper. Cook over low heat for about 30 minutes, until the onions are very tender and turning golden brown, stirring occasionally.
Remove from heat and stir in thyme. Set aside.
Preheat oven to 375 degrees.
Spray an 8-inch baking dish with nonstick cooking spray.
In a large saucepan melt remaining 3 tablespoons of butter and add flour and whisk for about 3 minutes to cook the flour.
Slowly add the milk and cream whisking while adding. Cook until mixture comes to a boil and has thickened. Stir in nutmeg.
Spread a few spoonsful of the sauce in the bottom of your prepared baking dish. Layer in the following order: lasagna noodles, onions, zucchini, sauce, and cheese repeat with the remaining ingredient.
Bake in preheated oven for 25-30 minutes until cheese is golden brown and the mixture is bubbly.
Let stand for 10 minutes and serve.
Perfect with a green salad and glass of wine.