Combine 7 cups water, the onion, garlic, potato, saffron, bay leaf, red-pepper flakes, and salt. Bring to a boil in a medium saucepan. Reduce heat to medium; simmer, partially covered, 30 minutes.
Pour soup through a fine sieve into a bowl, pressing on solids with a ladle to extract all the liquid and force through 1/2 cup solids. Return liquid and puree to pan; bring to a simmer over medium-high heat. Stir in sorrel; cook until wilted, about 3 minutes. Stir in lemon juice.
Preheat oven to 375 degrees. Arrange bread slices on a rimmed baking sheet. Using 1 teaspoon oil, lightly brush one side of each slice. Season with pepper. Bake until golden brown, about 8 minutes.
Divide soup among 4 bowls. Drizzle each serving with 1/2 teaspoon oil. Serve soup with baguette croutons.