2 tablespoons olive oil
6 large onions (2 white, 2 red, 2 yellow), halved and thinly sliced
6 cloves garlic, chopped
1 1/2 cups lowfat chicken or vegetable stock, divided
6 scallions, thinly sliced
1 tablespoon dried oregano
2 teaspoons salt (preferably sea salt), plus more to season
1 teaspoon freshly ground black pepper, plus more to season
9 whole-wheat lasagna noodles
2 packages (9 ounces each) frozen spinach
1 tablespoon salted butter
1 tablespoon all-purpose flour
2 cups part-skim ricotta
Grated zest of 1/2 lemon
Olive oil cooking spray
1 cup grated Parmesan, divided
Heat oven to 375°F. In a large sauté pan over medium heat, heat oil. Add onions and garlic; toss to coat.
Add 1 cup stock; cover and reduce heat; simmer until onions are soft, 20 minutes. Reserve 1/2 cup liquid. Add scallions, oregano, salt and pepper to onion mixture; increase heat to medium and cook, stirring, until liquid evaporates, 20 minutes.
Cook noodles as directed on package; drain and blot dry. Cook spinach as directed on package; drain, squeeze dry, and season with salt and pepper. In a small saucepan over medium heat, melt butter. Add flour; stir until roux is brown, 2 to 3 minutes. Add remaining 1/2 cup stock to reserved liquid; whisk into roux until thick, 5 minutes. In a bowl, beat egg; stir in ricotta and zest.
Coat a 9" x 13" pan with cooking spray; spread half of stock-roux mixture on bottom. Line pan with 3 noodles; layer on half of onion and ricotta mixtures, half of spinach and 1/3 cup Parmesan.
Repeat layer with 3 noodles, remaining onion and ricotta mixtures and spinach, and 1/3 cup Parmesan. Top with remaining 3 noodles, half of stock-roux mixture and 1/3 cup grated Parmesan.
Cover with foil; bake until light brown, 50 minutes. Reduce heat to 325°F; remove foil and bake 15 minutes more. Let lasagna sit 15 minutes; serve.