Whisk together all dressing ingredients in a small bowl; cover and chill until ready to use.
To prepare salad, bring water to a boil in a medium saucepan; stir in quinoa and simmer, covered, for 10 to 12 minutes. Remove from heat and let stand until all liquid has been absorbed. Fluff with a fork then stir in raisins; let cool. Transfer to a large bowl with celery, onion and thyme. Add dressing and toss well to coat. Cover and chill for at least 1 hour then stir in walnuts. (Salad may be made 24 hours in advance, adding walnuts just before serving.)
Remove a slice from the tops of the tomatoes and scoop out seeds with a small spoon. Place on individual salad plates and fill with salad, spooning any extra onto the plate around the tomatoes.