Preheat oven to 425F. Saute onions in 2 T butter until soft--about 5 minutes. Add the flour and stir to coat the onions. Gradually whisk in the milk and heavy cream stirring constantly to prevent lumps. Simmer for one minute.
Stir in Boursin cheese a little at a time until melted and smooth. Remove saucepan from heat and add spinach, parmesan, lemon zest, and seasonings. I use salt, pepper, a pinch of fresh grated nutmeg and about 1/4 teaspoon of cayenne.
Transfer spinach mixture to a prepared baking dish (I sprayed baking dish with Pam).
Combine crumbs, 2T. melted butter, oil, salt and pepper. Top spinach with crumbs, packing down to adhere.
Bake 20-25 minutes until crumbs are golden and sauce is bubbly.