Place ingredients in a bowl. Mix with a spoon until combined. Check for seasoning and add more if needed
Place filling in a pastry bag or a zip top baggie
For Manicotti
Boil salted water and add a teaspoon or 2 of oil
Cook Manicotti for 2 minutes less than what the box recommends
Remove carefully from water and place the manicotti to cool in a sheet pan
For Bechamel
In a saucepan, melt the butter over medium-low heat
When the butter is melted, add the flour and whisk until incorporated and the flour is cooked (about 45 seconds to 1 minute)
Add the warm milk and whisk vigorously to avoid lumps. Keep whisking until the sauce starts getting thick and creamy. Add the salt, pepper, and nutmeg.
Assembly
Spoon 1/3 of the sauce on the bottom of a rectangular baking pan
Pipe each cooled manicotti with the filling until the spinach reaches the other side. Place on the baking pan. Repeat with the rest of the pasta