Place one bag of frozen spinach in a covered glass container. Microwave on high for 8 minutes; spoon into a colander in the sink to drain. Repeat with second bag of spinach and add to same colander; use a large serving spoon to press excess water out of spinach. Place drained spinach in a serving bowl; cover to keep warm.

Meanwhile, melt butter in a medium saucepan over medium-low heat; stir in onion and kosher salt. Cook onion, stirring frequently, until soft and translucent, about 10 minutes. Add flour, nutmeg and cayenne; stir to combine. Slowly whisk in evaporated milk; cook, whisking constantly, until thick and bubbling, about 5 to 7 minutes. Pour sauce into a blender or food processor (or use an immersion blender in the pot); blend sauce until smooth.

Pour sauce over spinach and toss to combine; season with salt and black pepper. Yields about 3/4 cup per serving.