Heat the olive oil in a large stock pot over medium heat. Add the onion, garlic, madras curry, salt and pepper. Cook, stirring occasionally, until the onions start to soften, about 1 minute. Add the parsnips, sweet potatoes and red lentils and cook until warm, 1 to 2 minutes.
Add the vegetable stock and coconut milk. Simmer until the vegetables are tender and the lentils are cooked, 25 to 30 minutes. Stir in the vinegar and spinach and cook until the spinach is wilted, about 1 minute more. Ladle the stew into bowls and garnish with the Fresno chile, fresh mint and a lime wedge. Serve with grilled naan if desired.