Combine the shallot, thyme, parsley, mustard seeds, ½ cup of the oil, 1 cup water, ½ teaspoon salt, and ¼ teaspoon pepper in a large skillet. Bring to a low simmer.
Add the fish and cook, covered, until opaque throughout, 10 to 12 minutes.
Toss the frisée with the vinegar, the remaining 3 tablespoons of oil, ½ teaspoon salt, and ¼ teaspoon pepper in a large bowl.
Serve the fish over the frisée with the bread.