Spinach Lasagna

Ingredients

• 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
• 1 egg
• 2 cups part-skim ricotta
• Grated zest of 1/2 lemon
• Olive oil cooking spray
• 1 cup grated Parmesan, divided