Place all stir fry SAUCE ingredients together in a blender or food processor. Process or blend until smooth. Taste-test the stir fry sauce for sweetness - you want a balance of flavors: tangy, but definitely more sweet than sour. If needed, add 1 teaspoons to 1 tablespoons additional sugar. (Note: the sweetness of the sauce will depend on how sweet your mangos are.).
Prepare all other ingredients for stir-frying, and place next to the stove. If using fillets of fish, gently cut them into cubes. For shrimp, be sure to remove the shells. If you are using clams and/or mussels, rinse with cold water, removing any surface debris.
Pour 1 tablespoons canola oil into a wok or large frying pan and place over medium-heat heat. Add the garlic, onion, and snow peas. If using clams, add them now as well (Note: clams require a longer cooking time than mussels). Stir-fry 1-2 minutes, or until snow peas turn bright green. Add up to 1/3 of the stir-fry sauce when your wok or frying pan becomes dry.
Add the rest of the stir-fry sauce, plus the red pepper and seafood. Stir-fry gently, so that you don't break any shells. Also, if using fish fillets, these require extra gentleness, or they will break apart.
Stir-fry 5-7 minutes, or until all the seafood is cooked (mussels and/or clams should open after 7 minutes - if not, discard).
A minute or two just before serving, add the mango pieces (these just need warming).
Serve hot from the wok with rice or cooked barley on the side. Sprinkle with fresh coriander and decorate with lime wedges, if desired.