Preheat oven to 375°F; spray 13X9" pan with vegetable oil cooking spray.
Melt butter in a large, heavy skillet and cook onion over how heat until very soft, about 5 minutes. Add the garlic and cook about 1 minutes longer. Over medium-low heat, stir in the flour with a whisk, then gradually add the half-and-half, allowing the sauce to thicken slightly before adding more. When the sauce has thickened to about the consistency of whipping cream, add the Romano cheese and stir well. Add the salt and pepper and stir again. Add the scallops and cook for about 3-4 minutes, then add the shrimp and cook for 1-2 minutes or just until the shrimp turn pink. Turn off the heat.
Spoon about 2 tablespoons of the sauce from the skillet into the prepared pan. Cover the bottom of the pan with lasagna noodles, placing them side by side. Spoon 1/3 of sauce over the noodles, distributing half of the seafood evenly. Layer with more lasagna noodles, 1/3 of sauce, and remaining seafood. Cover with remaining noodles and the remaining sauce. Don't spoon any seafood on top of the noodles, just use sauce. Tuck in any edges of the noodles so they are all coated with sauce. Top with Parmesan.
Bake for 25 minutes, uncovered, or until bubbly and browned. Allow to sit for 10 minutes before serving.