1. Heat oil in a medium saucepan over medium heat. Cook onion until soft and translucent, about 5 minutes. Add garlic, and cook until soft, about 2 minutes. Add rice, and stir to coat. Add stock and water; bring to a simmer. Cook, covered, until rice is tender, about 20 minutes.

2. Prepare an ice-water bath. Working in 2 batches, puree soup in a blender until smooth. (For safety, remove cap from hole in lid, and cover with a dish towel to prevent spattering.) Transfer mixture to a bowl set in ice bath; let stand for 20 minutes, stirring occasionally. Stir in lemon juice, dill, and 1 1/2 teaspoons salt; season with pepper.