Sauté the Base: Warm the olive oil in a large, deep pan over moderate heat. Stir in the diced shallots and the sliced spring onions. Cook for about 7 minutes until soft and translucent, ensuring they do not brown.
Infuse Aromatics: Stir in the smashed garlic, chili flakes, and lemon zest. Cook for another minute until fragrant.
Simmer the Beans: Add the bay leaves and the drained butter beans to the pan. Pour in the stock and the fish sauce. Bring the mixture to a boil, then reduce the heat slightly to let the flavors meld for 5 minutes.
Steam the Clams: Turn the heat back up to high. Add the scrubbed littleneck clams and the splash of vermouth or wine. Cover the pan tightly with a lid.
Finish: Steam for 4–6 minutes or until the shells have opened. (Discard any that remain closed).
The Herb Reveal: Remove the lid and toss in the chopped lovage. The residual heat will release its distinct, savory aroma. Add a generous grinding of black pepper and give everything a gentle stir.
Serve: Ladle the clams, beans, and plenty of broth into deep soup bowls. Drizzle with a little high-quality olive oil and serve immediately with crusty bread for dipping.