Ingredients

• 2 large egg yolks*
• 2 tablespoons fresh lemon juice
• 1 tablespoon mustard powder
• 1/4 cup fresh basil leaves
• 1/4 cup fresh tarragon leaves plus 2 sprigs
• 3/4 cup extra-virgin olive oil plus more for serving
• 1 garlic clove, smashed
• 2 tablespoons kosher salt plus more
• 6 6-ounce arctic char steaks, each about 1 1/4" thick
• 2 bunches asparagus (about 1 1/2 pounds), trimmed
• Flaky sea salt (such as Maldon)

Instructions

1. Blend egg yolks, lemon juice, and mustard powder in a blender. Add basil and 1/4 cup tarragon. With machine running, drizzle in 3/4 cup oil; blend until smooth. Transfer mayonnaise to a small bowl, cover, and chill.
2. Combine tarragon sprigs, garlic, 2 tablespoons kosher salt, and 6 cups water in a large deep, lidded skillet and bring to a boil. Add fish, cover, and remove skillet from heat. Let stand until fish is just opaque in the center, 6-8 minutes.
3. Meanwhile, cook asparagus in a large pot of boiling salted water until crisp-tender, about 3 minutes; drain
4. Serve fish with asparagus; top with mayonnaise, drizzle with some oil, and sprinkle with sea salt.
5. DO AHEAD: Mayonnaise can be made 2 days ahead. Keep chilled.