Preheat oven to 350 degrees.
Place frozen spinach in a strainer and thaw. If pressed for time, run under hot water until thawed.
Heat oil in a large nonstick skillet over medium heat.
Add the onion and season with salt and pepper.
Cook until tender, 5-7 minutes. Remove from heat.
Meanwhile, use a rubber spatula and press all of the liquid out of the spinach until the spinach is dry.
Put the spinach in the food processor. Add the onion, breadcrumbs, Gruyere, parmesan, egg and butter. Process until completely combined and smooth.
Form the spinach mixture into 1-inch spheres. Place on a parchment lined baking sheet.
Bake until hot and lightly golden, about 12 minutes. Do not overcook or they will get dry.
Serve warm or at room temperature.
To make ahead: The spinach poppers can be prepared and refrigerated up to 1 day in advance or frozen up to 1 week in advance. Let thaw before baking.