In a medium bowl, combine the crab meat, mayonnaise, Dijon mustard, Old Bay seasoning, black pepper, and chopped green onions. Gently fold these ingredients to keep the crab chunky.

In a separate bowl, mix together the Gruyere and Cheddar cheese until well combined.

Take two slices of sourdough bread, sprinkle Gruyere on each, then spread half of the crab mixture on one and top with Cheddar cheese before placing the other slice on top.

In a skillet, melt one tablespoon of butter over medium heat. Cook the sandwich for 3-4 minutes until golden brown and crispy.

Flip the sandwich, add remaining butter, and cook for another 3-4 minutes until the second side is golden brown and cheeses are melted.

Remove the sandwich from the skillet, let cool for a minute, then cut in half and serve warm.