Whisk the buttermilk, light mayonnaise, Italian seasoning, dry mustard, garlic powder, salt, and pepper in a small bowl. Set aside.
Combine the lettuce, turkey, bacon, celery, and onions in a large bowl. Toss with the dressing, and season to taste. Cut each toast slice into 4 even triangles. Divide the salad among 4 plates, serve with the tomato wedges and toast triangles.