• Oven temperature: 375°F (190°C)
• HOMEMADE TOMATO SAUCE: Make as my quick pasta sauce (link in NOTES), except use a deep pot and cook the tomatoes slower and simmer for about 45 minutes.

FILLING:

• Begin by steaming the spinach in a closed pot with a little water, for about 5 minutes. Drain and cool, then squeeze the remaining liquid from the spinach and place in a large bowl.
• Add the ricotta, egg, salt and pepper and mix well.

BESCIAMELLA (WHITE) SAUCE:

• Melt the butter in a medium sized pan over medium heat, then add the flour and stir well. Continue to cook while stirring for about 3 to 4 minutes, but do not let it brown. (Cooking the flour well is important because the sauce will have a “raw-flour” taste if it isn’t cooked long enough.)
• Have the milk close by in a jug that is easy to pour from, and add a little drop of milk. The milk should sizzle and steam immediately, if it doesn’t, turn up the heat a little. Once the milk does sizzle, begin adding a little at a time, and stirring quickly to keep the sauce smooth.
• The secret to not having lumps in white sauce is to gradually increase the amount of milk you add each time; if you add too much at once, especially early on, chances are you will end up with lumpy sauce (which is completely reparable with an immersion blender)!
• Start with about 2 tablespoons of milk, then 2 more, then a little more each time. Once the sauce begins to form, you can add more milk each time. Just make sure all the milk has been absorbed after each addition, before adding more.
• When the sauce is finished, add salt, nutmeg and white pepper to taste, and remove from heat and keep a lid on the pot.

PASTA:

• Cook the pasta shells partially; I undercook them so that if you bite into one it should be about half-way cooked. Pasta water should be salted before cooking shells.
• Reserve about 2 cups of the pasta water, then drain most of the remaining water and immediately add cold water to cover the shells. This is to stop them cooking further.

ASSEMBLY:

• Put a little sauce in the bottom of a large, rectangular baking tray. Now you are ready to fill the shells. Using a spoon, hold a shell and fill it with some of the ricotta and spinach filling.
• Place the shell in the tray and continue with the rest of the shells and filling until they are all used. You may need another smaller dish if they don't all fit in the large tray. At this point, preheat the oven to 375°F (190°C).
• Heat the besciamella sauce, if it has become too stiff, then gradually pour over the tray(s) of stuffed shells. Don't flood the tray with white sauce. Add some of the pasta water to some of the pasta sauce in a bowl. Now, spoon the pasta sauce over the shells, but again, don't overload them.
• Finally, pour about a cup or so of the reserved pasta water in between the shells, so that they will continue to cook in the oven.
• Sprinkle with some freshly grated Parmigiano Reggiano, or Pecorino Romano cheese, then cover the tray with aluminum foil (If the foil touches the sauce, put a layer of parchment first so that the tomato sauce doesn't "eat" the foil.)
• Put into the preheated oven for about 25 minutes, then remove the foil and check to make sure that the shells aren't too dry (add more pasta water if it looks this way). Continue to bake for another 10 minutes or so, uncovered, then remove from the oven and replace the parchment and foil for about 10 minutes so that the shells absorb more of the liquid. This will also help them to hold together, when being cut.
• Serve the Ricotta and Spinach Stuffed Pasta Shells while still hot.