Thaw the frozen spinach in the refrigerator overnight. Before you use it, squeeze it as dry as you can using a dish towel.

Adjust oven rack to upper-middle position and heat oven to 425 degrees. Pulse bread in food processor to fine crumbs, about 10 pulses; transfer to bowl. Melt 3 tablespoons butter in microwave, about 30 seconds. Stir melted butter, ½ cup Gruyère, ½ teaspoon salt, and ½ teaspoon pepper into crumbs; set aside.

Melt remaining 5 tablespoons butter in Dutch oven over medium heat. Add onion and cook until lightly browned, 7 to 9 minutes. Stir in garlic and cayenne and cook until fragrant, about 30 seconds. Stir in flour and cook for 1 minute. Slowly whisk in half-and-half and bring to simmer. Cook, whisking occasionally, until slightly thickened and reduced to about 3½ cups, about 10 minutes.

Stir in curly-leaf spinach and cook until just wilted, about 30 seconds. Off heat, stir in mustard, nutmeg, 1½ teaspoons salt, 1 teaspoon pepper, and remaining 1½ cups Gruyère until cheese is melted. Stir in thawed frozen spinach until thoroughly combined. Season with salt and pepper to taste. Transfer mixture to 13 by 9-inch baking dish.

Sprinkle crumb mixture evenly over spinach mixture. Bake until bubbling around edges and crumbs are golden brown, 15 to 17 minutes. Transfer to wire rack. Combine parsley and lemon zest in bowl; sprinkle over gratin. Let cool for 10 minutes. Serve with lemon wedges.

TO MAKE AHEAD: Crumb mixture can be covered and refrigerated for up to 24 hours. Spinach mixture can be cooled and refrigerated for up to 24 hours. When ready to serve, bake gratin until hot throughout, 15 to 20 minutes, then proceed with step four.

Test Kitchen Discovery: Use a Combination of Fresh and Frozen