Whisk the sour cream, tarragon, lemon zest, and lemon juice in a small bowl until well combined and smooth.

Melt the butter in a large pot over medium-low heat. Add the leeks and a pinch of salt, cooking until they are softened and translucent, about 8-10 minutes—avoid browning them to keep the soup's color bright.

Stir in the garlic and cook until fragrant. Add the rice, vegetable broth, and water, then increase the heat to high and bring to a simmer.

Reduce the heat to low, cover the pot, and simmer until the rice grains are very tender and starting to fall apart, about 15-20 minutes.

Stir in the spinach and watercress one handful at a time until just wilted, which should take about 2 minutes; this short window ensures the greens stay vibrant rather than turning olive drab.

Process the soup in a blender in batches until completely smooth. Fill the blender jar only halfway and hold the lid down with a folded kitchen towel to prevent the steam from forcing the lid off.

Return the pureed soup to the pot and stir in the lemon juice. Season with salt and pepper to taste, then serve immediately with a dollop of the tarragon cream.