Creamy Spinach and Broccoli Lasagna

Heat oven to 425 degrees F. Heat the oil in a large skillet over medium-low heat. Add the onion, 1/4 teaspoon each salt and pepper and cook, covered, stirring occasionally, until very tender, 6 to 8 minutes. Stir in the garlic and cook for 1 minute. While the onion is cooking, thaw the spinach and broccoli according to package directions. Squeeze the spinach of excess moisture and pat the broccoli dry. Roughly chop both and place in a large bowl. Mix in the ricotta, 1 cup mozzarella, and 1/4 cup Romano. Add the milk to the onions and whisk to combine. Bring to a simmer, then whisk in the cream cheese, nutmeg, and remaining 1/4 cup Romano. Gently simmer (do not boil), stirring occasionally, until slightly thickened, 2 to 3 minutes. Spread 1/2 cup of the sauce on the bottom of an 8-inch square baking dish. Top with 2 1/2 noodles (breaking as necessary to fit). Spread a third (about 1/3 cup) of the remaining sauce over the top. Dollop with a third (about 1 1/3 cups) of the ricotta mixture. Repeat twice. Sprinkle with the remaining 1/2 cup mozzarella, cover tightly with foil and bake for 20 minutes. Uncover and bake until the noodles are tender (a sharp knife should go through with no resistance) and the top is golden brown, 8 to 10 minutes more. Let rest for at least 10 minutes before serving.