Wash and dry all produce. Preheat the oven to 400 degrees. Bring a large pot of salted water to a low simmer. Zest and halve the lemon. Thinly slice the scallions. Thinly slice the radishes. Finely chop the tarragon leaves.

Add the shrimp to the simmering water. Cook until just opaque, 2-3 minutes. Drain and rinse under cold water to cool.

Make the salad dressing: In a large bowl, whisk together 1 Tablespoon sour cream, half the tarragon, and a squeeze of lemon (to taste). Whisk in a large drizzle of oil. Season with salt and pepper.

Toast the brioche: Halve the brioche rolls and spread them with butter. Place in the oven to toast 3-4 minutes.

Dice the shrimp into 1/2-inch pieces. Toss in a medium bowl with the lemon zest, scallions, remaining tarragon, 2 Tablespoons mayonnaise, remaining sour cream, and a squeeze of lemon juice. Season with salt and pepper.

Toss the salad and serve: Toss the arugula and radishes into the salad dressing. Season with salt and pepper. Fill each brioche roll with shrimp salad, and top with a bit of arugula radish salad. Serve the remaining salad on the side. Enjoy!