Step 1: Make Your Vegan Buttermilk (5 Minutes)
In a jug, combine the plant milk with your chosen acid (lemon juice, apple cider vinegar, or white vinegar). Stir with a fork or whisk and let it sit for a minute. You'll notice the milk slightly thickening and curdling slightly—that's exactly what you want. This creates that tangy buttermilk flavor without dairy. Set it aside for now.
Step 2: Build Your Flavor Base (15 Minutes)
Put the spring onions into a large pan along with the diced potato, crushed garlic, and vegetable stock. Bring everything to a boil over medium-high heat. Once it's boiling, reduce the heat and simmer for 15 minutes, or until the potato becomes very soft and almost breaks apart when you stir it.
Why this timing? The potato needs to fully soften so that when you blend it, it creates creaminess. Undercooked potato won't blend smoothly and will create a grainy texture. You want it genuinely tender.
Step 3: Prepare Your Garnish Peas (2-3 Minutes)
While the base is simmering, put those 3 tablespoons of reserved peas into a separate small pot with water. Cook them for 2-3 minutes until just tender but still bright green. Drain them and immediately place them in a bowl of cold water. This stops the cooking and keeps them brilliant green. These are your garnish—they add visual appeal and a fresh, sweet burst of flavor.
Step 4: Add the Remaining Peas (3-4 Minutes)
After the potato has simmered for 15 minutes, add the remaining peas to the soup base. Simmer for 3-4 minutes. Don't overcook—you want the peas to stay bright green and maintain their fresh flavor. Overcooked peas turn gray and taste mushy. Precision here matters.
Step 5: Create the Magic (Blend and Season)
Stir in the mint, caster sugar, and lemon or lime juice. Cool the soup slightly (this prevents the blender from exploding), then blend to your preferred consistency. Some people love a completely smooth soup. Others prefer a slightly chunky texture. You're in control here—blend until it feels right to you.
Once blended, stir in half of the vegan buttermilk mixture. Taste and season generously with salt and pepper. The soup should taste bright, fresh, and slightly tangy.
Step 6: Decide: Hot or Cold?
Here's where this soup gets flexible. For serving warm: Simply reheat gently on the stovetop without boiling. (The buttermilk will curdle if you boil it—keep the heat gentle.)
For serving cold: Cool the soup quickly (you can place the pot in an ice bath), then chill completely in the refrigerator.
Step 7: Serve and Celebrate
Ladle the soup into bowls. Top each bowl with a small dollop of the remaining vegan buttermilk and scatter the reserved peas over the top. Add a crack of black pepper and maybe a tiny mint leaf if you're feeling fancy.
