In a large skillet sweat the onion in olive oil over moderate heat for 4-5 minutes.
Add spinach and season with salt and pepper; sauté for 5 minutes.
Remove from heat to cool down
Ricotta mixture: Combine ricotta and ¾ cup Parmigiano cheese; season with salt and pepper.
Prepare béchamel sauce: In a sauce pan melt butter and add flour, cook for 2-3 minutes. Add warm milk and bring to a simmer, whisk well so there are no clumps, season with salt and pepper. When the sauce comes up to a simmer cook for an additional 5 minutes.
Pan spray a 13” x 9” Lasagne dish. On the first layer spread ½ cup béchamel sauce.
Top with 4 Lasagna sheets, 1 cup ricotta mixture and a sprinkle of Parmigiano cheese Continue the same procedure for 3 more layers.
For the top layer spread remaining béchamel and ricotta mixture, sprinkle with Parmigiano cheese; bake for 35-40 minutes or until golden brown and bubbly on top.
Before slicing the Lasagne let it rest for 5 minutes.