For the dressing, whisk together all ingredients and then toss with onion. Set aside until ready to serve.
For the prawns, I like to season them with salt, pepper and paprika and then toss them with a bit of olive oil and minced garlic before throwing them on the grill, but you can certainly skip this step. Once you’ve grilled them, halve them lengthwise to top the salad.
Right before serving, toss arugula with dressing and plate. Nestle the wedges of tomato into the arugula and drizzle with leftover dressing. Place hot prawns on top and garnish with a nasturtium blossom.