Make the salad dressing. Combine ¼ cup olive oil, 2 Tbsp sherry vinegar, 2 Tbsp chopped tarragon, ½ tsp Dijon mustard, ½ tsp Kosher salt, and ¼ tsp black pepper in a measuring cup, then whisk to combine.
Optional: Add the onions. Raw red onions can have a sharp bite. To lessen their intensity, add the thinly sliced onion directly to the salad dressing while you make the couscous.
Make the couscous. Combine 1¼ cup water, 1 Tbsp olive oil, and ½ tsp Kosher salt in a medium saucepan, then bring to a boil. Stir in 1 cup couscous, then remove from heat and cover tightly. Let the couscous sit undisturbed for 5 minutes, then fluff with a fork. Transfer couscous to a large bowl. (You can also mix everything right in the saucepan!)
Dress the couscous salad. Add ¼ cup slivered almonds, red onion, 2 Tbsp chives, and chopped tomato, then pour the vinaigrette over the couscous. Stir to thoroughly coat, then taste and adjust seasoning as needed. Allow to cool to room temperature before serving. Meanwhile, cook the shrimp.
Season the shrimp. Preheat an oven to 425°F (220°C). Place shrimp on a rimmed baking sheet, then toss with 1 Tbsp olive oil, ½ tsp Kosher salt, ¼ tsp black pepper, and 1 Tbsp chopped tarragon. Make sure shrimp are in a single layer.
Cook the shrimp. Roast until just pink and opaque, 8-10 minutes. Shrimp can be served hot, room temperature, or cold.
Chop the shrimp. Once cool enough to handle, remove and discard the tails, then chop into bite-size pieces. I like to stir the shrimp directly into the couscous, but you can pile on top if preferred.
To serve: Divide the couscous between bowls, then drizzle with a little olive oil garnish with chives or tarragon, and finish with a pinch of flaky sea salt. Serve at room temperature or cold with lemon wedges on the side.