Prep the ingredients: Mince 1 onion and shred 7-8 oz imitation crab into strips with your hands.
Cook the onions: Add 3 tablespoons unsalted butter to a pan and turn the heat to medium. Once it melts, add the minced onions and fry until soft and translucent, about 5 minutes.
Make white sauce: Turn off the heat, add 6 tablespoons all-purpose flour, and mix well. Turn the heat to low-medium, and add 2 cups whole milk in 6-8 additions, mixing thoroughly each time. Simmer, stirring constantly, until it thickens enough to draw a line across the bottom of the pan, about 5-7 minutes.
Add crab and seasonings: Turn off the heat and add the crab, 1 teaspoon broth paste, ¼ teaspoon salt, ⅛ teaspoon ground black pepper, and ⅛ teaspoon nutmeg (optional) and mix well.
Chill the mixture: Transfer the white sauce to a tray (note 3), pressing plastic wrap against the surface to remove any gaps. Let it chill in the fridge for at least an hour.
Shape the croquettes: Divide the white sauce into portions and shape each into barrel shapes, ensuring you remove any air while shaping them. Chill in the freezer for 10 minutes.
Prepare the breading: In a small bowl, make the batter by whisking together 1 large egg, 2-3 tablespoons water, and ¼ cup all-purpose flour. The batter should be smooth and slightly thick but still have a runny consistency. Place 5 ounces panko bread crumbs in another small bowl or on a plate.
Bread the croquettes: Remove the croquettes from the freezer and dip each into the batter, then coat with panko, making sure to shake off any excess batter. Place them on a parchment-lined plate and chill in the freezer for 10 more minutes.
Prepare the oil (note 4): While letting the croquettes chill in the freezer, pour enough oil into a pot to cover at least half the height of the croquettes (ideally enough oil to cover all the height), and start heating it to 180°C (355°F) over medium-high heat.
Deep-fry the croquettes: Fry 2-4 croquettes at a time for about 2-3 minutes on each side or until golden brown. Be careful not to scrape the batter and panko when you flip them over. Place on a paper towel-lined plate to drain excess oil. Allow them to cool for about 3 minutes before serving to avoid burning tongues!