Halibut with Tarragon-Cream Sauce

This could work with any white fish--halibut is my favorite and is plentiful here, but I know it's very expensive in other places!
Try this with dill instead of tarragon for a different twist.
I served it with couscous & veggies on the side (and kinda mixed them together as I ate). It would also work well with pasta, potatoes, or rice (with veggies).
My kids gobbled this up--and we parents really enjoyed it too.
Serves 4

1 lb halibut, deboned and cut into smallish chunks (bite size more or less)
3 Tbs butter
3 Tbs flour or cornstarch
1-1 1/2 c milk
1/4 c parmesan cheese (this could be omitted, or another cheese could be substituted--I admit I used the cheap powdered type)
1 tsp salt
1/2 tsp pepper
2ish Tbs tarragon

Cut fish and set aside in a 9x9 baking dish.
In a saucepan over medium heat, melt butter and mix well with flour until it becomes smooth and thickens. Slowly add milk, about 1/4c at a time, and mix thoroughly after each addition. Add seasonings. Add cheese if desired. Stir continuously or at least very frequently so that it does not burn. Sauce will thicken as it cooks, so each time it starts to thicken you can add a little milk again until it reaches a nice thick cream sauce (it should be like gravy). Pour sauce over fish, stir to coat each piece of fish, then bake at 350 for about 10-15 minutes (stir once in the middle to make sure that all pieces of fish are cooking evenly).