Cook the frozen spinach according to the package directions. Drain thoroughly, then squeeze out as much moisture as possible. Set aside.

Melt 4 tablespoons butter in a 9- to 10-inch cast-iron skillet over medium heat. Add the shallots and cook until softened but not browned, about 4 to 5 minutes. Add the garlic and cook for about 30 seconds, until fragrant.

Sprinkle the flour over the shallots and garlic. Cook, stirring constantly, for about 2 minutes without browning.

Gradually whisk in the chicken stock until smooth, then slowly whisk in the half-and-half. Increase the heat to bring the sauce to a gentle simmer, about 2 to 3 minutes. Adjust the heat as necessary to maintain a gentle simmer and cook for about 5 minutes, stirring frequently, until thickened.

Stir in the Creole seasoning, kosher salt, white pepper, and nutmeg.

Fold the well-drained spinach into the sauce until evenly coated. Stir in ½ cup Romano cheese and cook for another 2 to 3 minutes, until the cheese has melted and the spinach is thoroughly heated. Taste and adjust the seasoning.

Preheat the broiler.

Melt the remaining 3 tablespoons butter. Combine the breadcrumbs, remaining ¼ cup Romano cheese, and melted butter, mixing until the breadcrumbs are evenly moistened.

Spread the breadcrumb mixture evenly over the spinach without pressing it into the surface.

Place the skillet under the broiler and broil for 2 to 5 minutes, watching carefully, until the topping is evenly golden brown.

Remove from the broiler and rest for 10 minutes before serving.