Preheat the oven to 450F. Arrange the peppers on 1 or 2 parchment or foil-lined baking sheets. Roast for 40 minutes, or until the skins are wrinkled and charred. Remove the peppers from the oven and allow them to cool and collapse (about 20-30 minutes). Remove the stems, skins, and seeds from the peppers; chop the peppers roughly and set them aside until you're ready to use. They can be transferred to an airtight container and stored in the fridge for up to a week.
Heat the oil or water in a large pot over medium high heat. Add the onion. Cook, stirring frequently, for 5-7 minutes, or until the onion is soft and clear. Add the garlic and cook for another minute or so. Add the potato, broth, and salt. Bring the mixture to a boil. Reduce the heat to a simmer, cover, and simmer for 20-25 minutes, or until the potatoes are soft.
Add the roasted peppers to the pot. Use an immersion blender to blend the soup till it's smooth and creamy, or transfer it to a blender in batches and blend till smooth. Transfer the soup back to the pot and heat gently. If the soup is thicker than you'd like, add some extra broth or water to achieve a desired consistency. Add the lime juice and crushed red pepper to taste; taste and adjust the seasonings as desired. Serve. Leftover soup will keep for up to 5 days in an airtight container in the fridge. It can be frozen for up to 6 weeks.