Put the oven rack in the middle position and preheat to 375 degrees F.
To make the topping, mix the panko, onion powder, celery seeds, salt and pepper until everything is evenly distributed, then drizzle on the olive oil and stir to combine.
½ cup panko, ¼ teaspoon onion powder, ¼ teaspoon celery seeds, ⅛ teaspoon salt, ground black pepper, 1 tablespoon olive oil
To make the mornay sauce, put the butter and flour in a small saucepan and cook over medium heat until the mixture is bubbly, but do not let the flour brown.
1 tablespoon cultured unsalted butter, 2 teaspoons all-purpose flour
Turn the heat down, then whisk in the cream until the mixture is free of lumps and it starts to thicken.
½ cups heavy cream
Once the sauce thickens, add the salt and stir in the cheese a little bit at a time until it is all incorporated. The mornay sauce will be very thick, but this is okay since the the tomatoes will release a lot of water.
¼ teaspoon salt, 50 grams Comté cheese
Arrange the sliced green tomatoes and mornay sauce in layers in an oven safe dish, then spread any remaining sauce on top of the green tomatoes.
300 grams green tomatoes
Sprinkle the panko topping evenly over the top of the tomatoes and place the dish in the oven. Bake for 30-35 minutes or until the sauce is bubbling, the tomatoes are tender, and the breadcrumbs are golden brown.