In a heavy-based saucepan over medium heat, melt half of the butter.
Stir in the flour and cook while stirring until the mixture bubbles.
Gradually add the milk, continuing to stir.
Cook for about 4 minutes, or until the mixture boils and thickens.
Remove the saucepan from heat and stir in half of the Gruyère cheese and the miso.
Season to taste.
Divide this cheese sauce between two 7¼-inch oval dishes that will fit in a 5- or 6-quart air fryer.
In a large saucepan, bring water to a boil.
Cook the broccoli for 1 minute or just until it turns bright green.
Drain the broccoli well and divide it between the two dishes.
Preheat the air fryer to 350°F for 3 minutes.
Melt the remaining butter.
In a small bowl, combine the bread crumbs, melted butter, and the remaining Gruyère cheese.
Mix them well and scatter this crumb mixture evenly over the broccoli.
Cover each dish with a piece of greased foil.
Carefully pull out the air-fryer pan and basket and place one of the dishes in the basket.
Slide the pan and basket back into the appliance.
Keep the temperature set at 350°F and set the timer for 20 minutes.
Leave it to cook.
Then, remove the foil from the dish and set the timer for an additional 5 minutes to cook until the broccoli is tender and the crumb topping is golden.
Transfer the dish to a wire rack and cover it with the reserved foil to keep it warm.
Repeat the cooking process with the remaining dish.
Serve the au gratins, topped with toasted flaked almonds.
Enjoy your Broccoli Au Gratin!
