Preheat the oven to 350°F (175°C). Grease a baking dish and set aside.

Cook the frozen spinach according to package instructions. Drain well, reserving ½ cup of the cooking liquid.

In a large skillet, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 3-5 minutes.

Stir in the flour, cooking for 1-2 minutes to eliminate the raw flour taste.

Gradually whisk in the evaporated milk and reserved spinach liquid, stirring continuously until the mixture thickens.

Add the Worcestershire sauce, garlic salt, celery salt, and black pepper. Mix well.

Stir in the shredded pepper jack cheese until melted and smooth.

Fold in the cooked spinach, ensuring it's evenly combined with the cheese mixture.

Transfer the spinach mixture to the prepared baking dish. Sprinkle breadcrumbs evenly over the top.

Bake for 20-25 minutes, or until the top is golden and the dish is heated through.