Melt 2 tbsp butter in a small saucepan on medium/low heat until it just starts to bubble. Add 2 tbsp flour and cook, stirring until smooth, about 2 minutes. Slowly add 1 1/2 cups milk, whisking continuously, cooking until thick. Remove from heat. Add the salt, pepper, and nutmeg. Stir in 1/4 cup grated Parmesan cheese and 1/4 cup of the grated Gruyère.
Cook the spinach, drain well, season with salt.
Precook the lasagna sheets in boiling water according to the package directions. (Normally, cooking time requires about 8 minutes. The lasagna should not be thoroughly cooked.)
Dry on a clean cloth.
Mix 5 oz. of pistachios with grated Parmesan, 7 tablespoons of olive oil and some salt into a smooth sauce. Make a thick béchamel sauce.
Cut the Brie into small pieces.
Grease a baking dish with 1 tablespoon of olive oil. Add the first lasagna sheet.
Layer the spinach, a little of the pistachio and the béchamel sauce, as well as the slices of Brie.
Repeat until all the ingredients are added, ending with the béchamel sauce.
Sprinkle the top of the lasagna with the remaining pistachios, crushed coarsely in advance.
Place in an oven, preheated to 400° F and cook for 35 min.