In a large pot, heat the olive oil over medium heat and sauté the onion with a pinch of salt for about 5 minutes, until soft and translucent. Add the cauliflower and broth, raise the heat to high, and bring to a boil. Lower the heat to medium and simmer for about 10 minutes, until the cauliflower is just tender. Add the lettuce, peas, and 1 teaspoon of salt and simmer for about 2 minutes, until the peas are just cooked but still vibrant green. Remove the pot from the heat and allow to cool slightly. Add the mint.