In a small skillet, toast the curry powder over moderate heat, stirring, until fragrant, about 30 seconds. Scrape it into a bowl. Add the mayonnaise, apple, dill, onion, celery, scallion and lemon juice. Flake the tuna into the bowl and mix well. Season with salt and pepper. Cover and refrigerate for at least 30 minutes. To serve, mound the arugula and tomatoes on 4 plates. Set an avocado half on top and fill with the tuna.
Use Hass avocados; they're easy to peel and hold their shape beautifully.