Begin by melting the margarine in a 2-quart saucepan. Use a fork or a whisk to stir in the flour making sure it is smooth. Gradually stir in the milk. If you have a whisk you can go a little faster because a whisk can beat out any lumps. If you are using a fork then go very slowly so you can keep the mixture smooth. When the mixture is smooth stir in the sugar, salt and pepper. Cook and stir the sauce until it begins to boil. Allow it to boil for a full minute by the clock. Now remove the sauce from the heat.
Drain the peas and the tuna. Stir gently and reheat only till boiling again. Do not overcook or the peas will get mushy. Serve over buttered toast or biscuits, rice, or noodles. The salt may be replaced with celery salt or garlic salt if desired.
To use fozen peas instead of canned, simply replace the canned peas with 2 cups frozen peas. Allow the sauce to reheat for a minute or two so you will be sure the peas are cooked through. Do not ovecook the frozen peas wither. This is vital for a good dish.