In a bowl, whisk together the lemon juice, garlic, sugar and 2 teaspoons salt. Whisk in the olive oil. Pour 1/2 cup of the dressing into a large bowl. Add the celery, beans and parsley, toss well and season with pepper.
Add the tuna to the bowl with the remaining dressing, flake the fish and toss to coat. Season to taste with salt and pepper.
Divide the celery salad among 4 plates and top each portion with tuna.