Place frozen spinach and water in a saucepan over high heat; bring to a full boil. Reduce heat to medium, cover, and cook for 8 minutes. Separate with a fork and cook for 2 more minutes. Drain, reserving 1/2 cup of the cooking liquid.

Melt butter in a saucepan over low heat. Add flour, stirring until blended and smooth, but not brown, 1 to 2 minutes. Add onion and cook until soft but not brown, 5 to 7 minutes.

Slowly add evaporated milk and reserved cooking liquid, stirring constantly to avoid lumps. Cook, stirring constantly, until smooth and thick, 3 to 5 minutes. Add cheese spread, celery salt, garlic salt, pepper, cayenne, and salt; cook and stir until melted, about 2 to 3 minutes. Stir in cooked spinach.

Stir in cooked spinach and serve immediately.