Step 1

Preheat oven to 425°. Place mahi-mahi on a rimmed baking sheet and rub all over with 1 Tbsp. oil; season on both sides with salt and pepper. Brush or rub fish on both sides with 1 Tbsp. plus 1 tsp. mustard. Using a Microplane, finely grate zest of lemon over fish. Set lemon aside.

Step 2

Bake mahi-mahi until opaque throughout and flesh flakes easily with a fork, 8–10 minutes.

Step 3

Meanwhile, heat 1 Tbsp. oil in a large skillet, preferably cast iron, over high. Cook asparagus, turning occasionally, until blistered in spots, about 4 minutes total; season with salt.

Step 4

Remove mahi-mahi from oven. Cut reserved lemon in half and squeeze juice from 1 half over fish. Squeeze juice from remaining half into a medium bowl (catching any seeds with your other hand). Add honey, remaining 1 tsp. mustard, and remaining ¼ cup oil and whisk until smooth; season dressing with salt and pepper.

Step 5

Add avocado and arugula to dressing and toss gently to coat. Taste salad and season with more salt and pepper.

Step 6

Divide fish and asparagus among plates and scatter salad on top.