Preheat oven to 425F

Saute onions in 1 Tbsp butter until soft, about 5 min. Add the flour and stir to coat the onion.

Gradually whisk in the milk and heavy cream, constantly stirring to prevent lumps. Simmer for 1 minute..

Stir in Goat cheese a little at a time until melted and smooth.

Remove the saucepan from the heat. Add spinach and seasonings.

Transfer spinach to the prepared baking dish. Combine crumbs with Parmesan cheese and sprinkle over the top of the casserole, packing down to adhere.

Dot with the remaining butter

Cook 20 to 25 minutes until crumbs are golden and the sauce is bubbly