This was one of the first dishes I learned to make in High School.  Back then it was your basic white sauce recipe, which was traditionally made with the tuna, peas, and then poured over baking powder biscuits.
Well, we only use Soy milk in this house, (unless when I make SCD cream) and after years of not having white sauce, I began playing around.  The key to a decent non dairy white sauce, is to find the most neutral unsweetened dairy alternative.  We have found that Natura Unsweetened Soy Milk is really neutral tasting, and works surprisingly well for a white sauce.  The other trick is to "up" the flavor of the white sauce.  This is a good sauce for many casseroles as well.  We had this served over my Parmesan Cheddar Quick Bread which I had made in the morning, then reheated in the oven just to a warm temperature.
2 1/2 cups Natura Soy Milk (or regular milk if you can have dairy)
3 Tblsp. corn starch
3 rounded Tblsp. butter
1 small finely diced celery stalk, with leaves
1 large finely minced shallot
2 minced garlic cloves
2 tsp. fresh minced Tarragon (or use 1 tsp. dried)
salt and pepper to taste
1 small can of white flaked tuna, drained (this would be tasty with Salmon too)
1 cup frozen baby peas, defrosted
1/2-2/3 cup Parmesan Cheese
Melt the butter in a sauce pan on low heat.  Add in the minced shallot, celery, garlic, salt and pepper, then turn down to minimum heat and let simmer about 10 minutes.  Add in the minced Tarragon, sauteing a few minutes.
Pour a little of the Soy Milk into a cup and add the corn starch to make a slurry - it should be somewhat on the thick side.
Pour in the rest of the Soy Milk into the pan of shallots, and celery, stirring to combine.  Bring to a light simmer, and drizzle in the corn starch slurry, whisking, until thickened.  Add in the defrosted peas, and drained tuna.  Stir to combine, until it is back up to a light simmer.  Remove from heat, and add in the Parmesan Cheese, stirring until melted, and pour over biscuits or other quick bread.