Preheat oven to 425°F. Lightly butter or oil a shallow, 2-qt. gratin dish ( such as an 8-inch).

Melt the butter in a large, heavy-bottomed saucepan over medium heat. Add onion and a few pinches of salt and pepper and cook 4 to 5 minutes, stirring occasionally, until softened. Stir in flour and cook 1 minute, stirring constantly. Gradually pour in broth and milk while stirring, to avoid lumps. Increase heat to medium-high, add a few pinches of salt, and simmer about 5 minutes, stirring often, until the sauce lightly coats the back of a wooden spoon. Stir in 1/4 cup Gruyere until melted, then stir in nutmeg and spinach until evenly combined. Season to taste with salt and pepper.

Spoon into the prepared baking dish and smooth the top. Sprinkle with remaining 1/2 cup Gruyere. In a small bowl, mix together breadcrumbs and olive oil; sprinkle on top. Bake 20 to 25 minutes until breadcrumbs have browned and the edges of the gratin are bubbling.