Melt the butter in a large Dutch oven or soup pot over medium heat. Add the diced onion and cook for about 5 minutes, stirring occasionally, until softened and translucent but not browned.
Stir in the minced garlic and cook for another minute until fragrant. The kitchen should smell like warm onions—this is when you know it's ready to move forward.
Pour in the vegetable broth and bring to a simmer. Add the watercress and peas, stirring gently to submerge everything. Cook for 8-10 minutes until the greens have wilted completely and the peas are tender.
Working in batches, carefully transfer the soup to a blender or use an immersion blender directly in the pot. Blend until smooth—be careful of hot steam when opening a regular blender lid. Return the blended soup to the pot if using batches.
Stir in the heavy cream and lemon juice. Season with salt and pepper to taste—the soup should have a bright, fresh flavor that isn't overpowered by any single ingredient. Taste and adjust as needed before serving.
