Melt butter in heavy medium saucepan over medium low heat. Add onion and garlic and saute onion until translucent, about 8 minutes. Add flour and cook 2 minutes, stirring occasionally. Gradually whisk in milk. Simmer until mixture thickens, stirring constantly, about 2 minutes. Mix in lemon peel and nutmeg. Remove from heat. Season with salt and pepper. (Can be prepared 1 day ahead. Cover white sauce and refrigerate.) Grease 20 cm square baking pan. Steam asparagus over boiling salted water until just crisp tender, about 6 minutes. Refresh under cold water; drain. Cut asparagus into 3 cm pieces. Combine asparagus and ricotta in small bowl. Cook lasagne noodles in large pot of rapidly boiling salted water until just tender but still firm to bite, stirring occasionally. Drain. Rinse noodles with cold water and drain well. Spread half of white sauce in bottom of pan. Top with single layer of lasagne noodles, trimming to fit. Cover with half cup white sauce. Top with half of asparagus mixture. Top with half each of Swiss cheese and bacon. Top with another layer of noodles. Cover with remaining white sauce. Top with remaining asparagus mixture, Swiss cheese and bacon. Cover with remaining noodles. Drizzle top with tomato sauce. Sprinkle with Parmesan cheese. (Can be prepared 1 day ahead. Cover and refrigerate. Bring to room temperature before continuing.) Preheat oven to 350 degrees. Bake lasagne until heated through, about 20 minutes. Cool slightly before serving. 6 servings.