Tangy Flounder or Sole (Hl)

Yield:	4 Servings
Categories:	Seafood, Grilling

1 1/4	lb	Flounder or sole fillet (1/8 inch to 1/4 inch thick)
1/4	c	Balsamic vinegar
1/4	c	Lemon juice
1	ts	Dried mustard powder
1	sm	Yellow onion, chopped
2	ts	Fresh or 1 tsp dried parsley
		Lemon slices, optional