1 tbsp olive oil
1/4 cup all-purpose flour
1 large onion, chopped
2 1/4 cups whole milk
2 celery ribs, chopped
3/4 cup ricotta
1 large carrot, peeled and chopped
salt and freshly ground black pepper
2 oz pancetta, diced
1/4 tsp freshly ground nutmeg
1 lb ground beef round
8 oz (12 sheets) no-boil lasagna
1 14.5 oz (411 g) can chopped tomatoes with their juices
1/2 cup freshly grated Parmesan cheese
1 tsp dried oregano
1/3 cup hearty red wine
4 tbsp butter
To make the ragu sauce, heat the oil in a large saucepan over medium heat. Add the onion, celery, carrot, and pancetta and cook for five minutes, until softened. Add the beef and cook, breaking up with a spoon, about 10 minutes, until browned.
Add the tomatoes with their juices, the wine, and the oregano. Bring to a boil, reduce heat to low, and simmer for at least 40 minutes and up to 2 1/2 hours.
Meanwhile, make the white sauce. Melt the butter in a saucepan over low heat, whisk in the flour, and cook for a minute. Whisk in the milk. Cook, whisking often, until the sauce boils. Stir in the ricotta cheese and season with salt and pepper and nutmeg.
Preheat the oven to 375F. Oil a large rectangular (9 x 13 in) baking dish and spread 1/4 cup of the white sauce on the bottom. Arrange 3 lasagna sheets in the dish and top with one third of the ragu and 1/2 cup of the white sauce.
Repeat layers twice more. Top with 3 lasagna sheets, the remaining white sauce, and the Parmesan cheese.
Bake for 45 minutes, until the top is golden and bubbling. Let stand 10 minutes, then serve hot.