3 tbsp butter
3 tbsp flour
1 c whole milk, heated
Non-stick vegetable spray
2 (10-oz) packages frozen chopped spinach, thawed and thoroughly squeezed to remove liquid
1 c mayonnaise
6 Large eggs, well beaten
1/4 c minced sweet onion
2 tbsp grated Parmesan cheese
1/4 tsp salt
1/4 tsp freshly ground black pepper
Melt butter in small heavy saucepan over medium-low heat. When the butter begins to foam, gradually whisk in flour to make a roux: stir continually so the roux cooks. Reduce heat and continue to stir and cook until the roux is a light golden color. Remove pan from heat. Gradually whisk in heated milk. Return pan to heat and whisk until mixture is smooth, simmering and thickened. Remove from heat.
Preheat oven to 350 degrees F. Spray a 2-quart soufflé dish or straight-sided casserole (that allows room for expansion) with non-stick vegetable spray.
In a bowl, combine squeezed spinach, mayonnaise, eggs, onion, Parmesan cheese, salt and pepper. Fold in the sauce and mix thoroughly.
Spoon the mixture in to the prepared dish. Bake for 45 minutes in preheated oven. Serve immediately.