Preheat your oven to 180 C. get out and lightly grease a 9x13 baking dish.
Bring a pot of water to boiling and add the shrimp (remember, no tails) and cook until just pink, about 2-3 minutes, remove from heat and drain.
Season the fillets lightly with salt and pepper to your liking then lay them in a single layer in the baking dish, sprinkle some thyme and parsley over the fish, then place the shrimp in a single layer over of the fish, top with a sprinkling of thyme, rosemary, and salt to your liking.
In a small sauce pan, melt the butter over medium heat then whisk in the flour and salt to make a paste, slowly whisk in the milk and stir until the sauce thickens to coat the back of a spoon, remove from heat and pour over the shrimp in the baking dish. Sprinkle the Parmesan cheese on top of the sauce, bake uncovered for 20-25 minutes until hot and bubbly. Remove from the oven, let this rest for 10 minutes, sprinkle the chopped green onion over the top.
Serve with a tomato salad or a potato salad.