2 tablespoons of extra-virgin olive oil1 yellow or white onion, chopped1 (14 oz) potato, peeled and diced2 medium carrots, scrubbed well and diced4 cloves of garlic, minced5 and 1/2 cups no-salt-added vegetable broth, heated1 teaspoon of sea salt, or more as needed1/4 teaspoon of freshly ground black pepper, or more as needed5 oz of fresh watercress2 tablespoons of unsalted butter (optional