Start by cooking the 2 10oz packages of frozen, chopped spinach according to package directions. I recommend using whatever stove-top directions are given instead of microwave directions. Once cooked, drain the spinach over a fine mesh strainer set over a saucepan and reserve the liquid.

While the spinach is cooking, toast the breadcrumbs. Set a small skillet over medium-low heat and melt 1 tablespoon butter. Add ¼ cup panko and toast, tossing occasionally, until it becomes light golden-brown, about 3-4 minutes. Once they get some color, remove from the pan and set aside to cool.

Once the cooked spinach has cooled a bit, start on the rest. Set a large pot over medium heat and melt ¼ cup unsalted butter. Toss in the diced shallot and cook until softened but not browned, about 2-3 minutes. Add the 2 cloves of minced garlic and stir to combine for about 30 seconds.

After the shallot and garlic has been sauteed, add ¼ cup all-purpose flour and stir constantly to create a blonde roux. This should only take about 2-3 minutes to toast the flour and make the roux.

Pour the ¾ cup half-and-half into a measuring glass and add a ½ cup of the reserved spinach cooking liquid. Slowly pour this into the roux, whisking constantly to form a thick bechamel sauce.

To this sauce, add 6oz of shredded pepper Jack cheese, ½ teaspoon salt, ½ teaspoon ground black pepper, ¾ teaspoon celery salt, ½ teaspoon garlic powder, ¼ teaspoon ground cayenne, 1 teaspoon Worcestershire sauce, and ½ teaspoon of hot sauce if using. Stir to combine and continue cooking on medium-low heat until the cheese has fully melted. Keep stirring frequently during this process.

Once the cheese is fully melted, add the cooled spinach and stir together until fully combined with the cheese sauce. Transfer to a casserole dish and sprinkle the top with the reserved, buttered breadcrumbs and serve warm.