Pre-heat your oven to 180°C. Remove any excess liquid from the spinach by squeezing it out, then roughly chop it and put it in a bowl with four tablespoons of freshly grated Parmesan, the ricotta, season with salt and pepper and then mix them together. Then add in the crushed garlic and combine, put the mixture to one side until it's needed.
Place one slice of beef onto a clean board and beat it out, making sure that it's flat. Lay a slice of salami on top & spoon in a heaped tablespoon of the mix in a pile, about 3cm from one end. Put a fresh basil leaf on top and roll it up, starting at the end of the filling. Season the top with salt and pepper and then repeat until all of the beef slice are used up.
Put a large pan on the the with some olive oil in, then put the rolls into the pan, putting the sealed side down and lightly brown all over. Do not turn them over until the join is sealed. In an oven-proof dish lay half of the tomato sauce then put rolls on top and cover with more sauce.
For the cheese sauce, melt the butter in a pan, stir in the flour until it forms a paste, then allow it to simmer for 2 minutes, to cook out the flour taste. Gradually add the milk whilst whisking continuously so it forms a smooth sauce, add the nutmeg. When the mixture begins to boil, reduce the heat immediately and simmer for 10 to 12 minutes, stirring every so often so that the sauce does not stick. Remove from the heat and grate in 3 heaped tablespoons of Parmesan, stir it all together allowing the cheese to melt slightly.
Pour the sauce over the beef rolls, then using a knife or the handle of a spoon, ripple together the cheese sauce and the tomato sauce. Grate over some more Parmesan, add a few sprigs of fresh rosemary and cook in the oven for 25 minutes.