How to Make It
Preheat oven to 375°F. Bring a small saucepan of water to a boil over high. Add mustard seeds, and reduce heat to medium; simmer until tender, about 8 minutes. Drain using a fine wire-mesh strainer; transfer mustard seeds to a small bowl. Whisk in 1/4 cup oil, lemon juice, Dijon, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Brush salmon tails all over with remaining 2 tablespoons oil; sprinkle with remaining 1 teaspoon salt and remaining 3/4 teaspoon pepper. Arrange tails evenly on a baking sheet. Roast in preheated oven until tails are lightly charred and meat pulls away with a fork, 8 to 10 minutes, flipping halfway through cook time. Increase oven temperature to broil; broil until tails and skin are charred, about 3 minutes. Transfer tails to plates.
Toss together mixed herbs and half of the mustard dressing in a large bowl; season with salt and pepper to taste. Divide salad among plates with salmon. Serve immediately, passing remaining dressing at the table.