Dice carrots, celery and onion.  Saute gently in olive oil with a little salt and pepper to taste.  Saute until all ingredients soften, but try not to brown.

Layer a 10x6 deep baking dis with the sauteed vegetables.  

Add spinach to the same pan.  Cook with a touch more olive oil, salt and pepper until wilted.  Layer over sauteed vegetables in baking dish.

Add your choice of cooked fish on top of the wilted spinach layer.  In this recipe we're using smoked salmon, so its as simple as opening the packet, and breaking up over the dish.

Make a béchamel sauce.  Gently melt butter in a sauce pan.  When melted whisk in flour.  Next add warmed milk slowly, while whisking.  Bring to a boil and reduce heat.  Mix in Dijon mustard and heavy cream.    Season with salt and pepper and pour over salmon.

Make mashed potatoes.  Be sure to add plenty of butter and milk to your taters, we're going for rich!  When made, fork careful over béchamel layer in baking dish.  Fluff mashed potato with fork for little peaks.

Bake in oven at 400f for 20 to 30 minutes.  I usually cook until I see the béchamel bubbling up through the mashed potatoes.

Turn oven off, broil the pie for 5 to 10 minutes until the fluffed peaks brown, much like a shepherds pies.

Allow to cool and serve.  Dish goes really well with some quality crisp French bread or similar.