Finely dice 1 medium onion (about 1 ½ cups). Finely grate 2 garlic cloves on a Microplane. Peel and finely grate 1 (1-inch) piece fresh ginger on a Microplane (about 2 teaspoons).
Heat 3 tablespoons olive oil in a Dutch oven over medium heat until shimmering. Add the onion and cook, stirring occasionally, until softened, 3 to 5 minutes. Add the garlic and ginger and cook until fragrant, about 30 seconds. Add 1 (15-ounce) can pumpkin puree, 3 tablespoons red Thai curry paste, 2 teaspoons kosher salt, and ½ teaspoon black pepper, and stir to combine. Cook, stirring occasionally, until the mixture is slightly thickened and sticks to the bottom of the pot, 4 to 5 minutes.
Add 1 cup dried red lentils, 6 cups low-sodium vegetable broth, and 1 (about 13-ounce) can unsweetened coconut milk, and stir to combine. Bring to a boil over medium-high heat. Reduce the heat to maintain a simmer and cook, stirring occasionally, until the lentils are soft and the flavors meld, 30 minutes.
If desired, for a creamier soup, you can use an immersion blender or transfer to a blender to puree to your desired consistency. Juice 1 medium lime into the soup and stir to combine.
