Melt 1 tablespoon butter in large saucepan. Add onion and garlic and cook to soften
Add remainder of butter and melt. Add flour and whisk until smooth paste. Cook on low for about 2-3 minutes.
Add warmed milk and chicken stock, all at once. Whisk, or stir until thickened over medium heat. (about 20 minutes). 'once thickened add Parmesan and pepper. Once that is melted, adjust salt to taste. Set aside.
For Assembly
Preheat oven to 350°F.
Prepare ricotta mixture. Boil lasagna noodles and drain. Work quickly now, so that lasagna doesn't stick together.
Add a few tablespoons on sauce to pan, sprayed with no stick spray. Place layer of lasagna in a lasagna pan. Mine is 14.5" x 10.5", so if fits 5 noodles per layer. Yours may vary.
Spread half of ricotta mixture on lasagna. Top with another layer of noodles.
Spread about 1/3 of bechamel on top of pasta. Sprinkle with half of the sausage and half of the Romanesco. Repeat layers.
On top of last layer of pasta, spread with the rest of the bechamel and then with the sliced, or grated mozzarella.
Bake for 1 hour, or so until hot and bubbly. Let rest 15 minutes before slicing.
Video
Notes
Cover lasagna with foil if the cheese on top is getting too browned.Can be made 1-2 days ahead. Refrigerate. Allow the pan to come to room temperature for at least 30-45 minutes, before baking. Bake at 300°F for 30 minutes. Then increase oven temperature to 350°F. Cook until hot and bubbly.If lasagna is covered, don't let the foil touch the top of the cheese. It will stick to foil.