Melt the butter in a medium, microwavable bowl, remove and let the butter cool but not harden.
In a food processor, pulse 4 slices bread (dry is best) until the crumbs are between the size of sand and small gravel. If the breadcrumbs are too moist, spread them out onto a rimmed baking sheet and place in a preheated 300°F oven for 5-10 minutes. The crumbs should be dry but not cooked at all since they will brown with the gratin. Watch closely.
Combine the melted butter, dry breadcrumbs and Parmesan; set aside.
For the Gratin:
Heat oven to 425°F and spray a 1½ quart gratin or baking dish with nonstick spray.
Squeeze all excess water from the thawed spinach, being sure almost all the water is removed.
Heat a large skillet and add the oil and butter over medium heat. When butter quits bubbling, add the onions and cook 7-9 minutes until they translucent and tender.
Sprinkle the sautéed onions with the flour and nutmeg and cook 2 more minutes, stirring continuously.
Reduce heat to medium-low and add the cream and half ‘n half (or milk). Stir constantly until mixture thickens.
Add the spinach (after squeezing out all excess water) and stir.
Add ¼ cup Parmesan, salt, pepper and crushed red pepper. Mix well.
Transfer the spinach mixture to the prepared dish and sprinkle with the remaining ¼ cup Parmesan and the Gruyere.
Bake the gratin at 425°F, for 10 minutes. Sprinkle the top with the crumb topping and bake an additional 10 minutes or until hot and bubbly.