Slow-Roasted Arctic Char with Lemon-Mustard Seed Topping

1 tablespoon extra virgin olive oil

1 1/2 tablespoons multigrain bread crumbs

1 tablespoon finely chopped fresh flat-leaf parsley

1/4 teaspoon dried tarragon

2 teaspoons mustard seeds, crushed

1 tablespoon white wine vinegar

1 tablespoon fresh lemon juice

2 teaspoons Dijon mustard

teaspoon sugar substitute

Four 5-ounce Arctic char fillets, skin on

Freshly ground pepper