Preheat the oven to 350º F.

Butter and flour a 15-by-10 inch rimmed baking sheet. Line the baking sheet with parchment paper. Butter and flour the parchement paper, shaking off any excess flour.

In a small saucepan over medium heat, melt the butter and when bubbling add the flour and cook, stirring, until a blonde roux is formed, about 3 minutes. Slowly add the milk, whisking until the mixture begins to thicken and bubble. Reduce the heat and simmer until thick, about 2 to 3 minutes. Transfer the mixture to a large bowl and slowly whisk in the egg yolks, one at a time. Add the parmesan cheese and season with the salt and pepper.

In the bowl of a standing mixer or in a large bowl using an electric mixer beat the egg whites until frothy. Add the cream of tartar and continue whisking until the egg whites form a soft peak. Stir in 1/4 of the egg whites into the yolk mixture to lighten it. Gently, fold the remaining egg whites into the batter. Pour the batter into the prepared baking sheet and gently smooth the top with an offset spatula.

Bake in the middle of the oven for 15 minutes, rotating the pan halfway through cooking. The eggs should be golden brown on top and feel firm to the touch. Transfer the pan to a rack and cool for several minutes.

In a medium size saute pan over medium heat add the proscuitto and butter and cook until the proscuitto is crisp. Add the spinach and herbs and continue to cook until the spinach is wilted and most of the liquid has evaporated. Set aside to cool for several minutes. Drain off any excess liquid from the pan. Add the spinach to a bowl with the Boursin cheese and cream together until well combined. Set aside until ready to use. The spinach mixture can be made a day in advance and brought to room temperature when ready to use.

Once the eggs have cooled slightly, butter another piece of parchment paper cut to the same size as the baking sheet. Place it on topo of the egg mixture. Place a damp kitchen towel over the parchment and then carefully invert the eggs onto a clean, dry work surface. Gently peel the parchment off of the egg mixture, being careful not to break or tear it. Invert the eggs again and remove the parchment from the other side so the eggs are resting only on the damp kitchen towel.

Using an offset spatula, spread the spinach mixture gently over the eggs, leaving about 1/2-inch border around the edges. Starting on the long side, use the towel to gently roll up the roulade, like a jelly roll. Carefully, transfer the roulade to a platter to serve. Cut into 2-inch thick slices and serve immediately. If you will not serve immediately, you may reheat the roulade in the microwave for about 10 seconds before serving.

Serve the roulade with Butter Braised Spring Vegetables, a salad or even a dollop of sour cream and caviar.