This is easy and so delicious. You can use salmon instead of trout if you want.
1 lb fresh trout our salmon fillets
1/2 cup good quality balsamic vinegar
2 heaping tbls dijon mustard
2 garlic cloves, minced
Heat vinegar over medium low heat and simmer until it reduces by about half. Remove from heat. Stir in mustard and garlic. If it is too thick, add a splash or two of additional balsamic vinegar. Brush onto fish and bake in over at 400 for 10-15 minutes (depending on the thickness of your fillets).