Nutty Gruyere cheese and crunchy toasted breadcrumbs take the classic steakhouse side to the next level. It'll quickly become a family favorite.
In a large pot, bring 1 1/2 cups water and 1 teaspoon salt to a simmer. Add spinach; cover and cook until wilted, about 2 minutes. Drain in a colander, pressing out as much liquid as possible with the back of a wooden spoon; coarsely chop.
In a medium saucepan, melt 2 tablespoons butter over medium. Add shallot and cook, stirring, until softened, about 4 minutes. Add flour; cook, stirring, until mixture is pale golden and has a slightly nutty aroma, 2 to 3 minutes.
Pour in milk, whisking constantly, until smooth. Cook, stirring constantly along bottom of pan, until bubbling and thickened, 6 to 8 minutes. Remove from heat; add mustard and 1/2 cup cheese. Whisk until smooth.
In a large bowl, stir together spinach and cheese sauce until spinach is completely coated. Season with salt and pepper.
Heat a medium skillet over medium. Add remaining 1 tablespoon butter and panko; cook, stirring constantly, until golden brown, about 5 minutes. Season with salt and pepper.
Preheat oven to 400 degrees. Lightly brush a 7-by-11-inch oval gratin dish with butter. Transfer spinach mixture to dish; sprinkle top with toasted panko and remaining 1/4 cup cheese. Bake until browned on top and edges are starting to bubble, about 20 minutes. Serve immediately.