First, cook the sweet potatoes for the falafel. Pierce flesh with the tines of a fork and either bake in the oven wrapped with aluminum foil for about 45-60 minutes, or microwave wrapped in a paper towel for 7-10 minutes. Remove from oven or microwave and let sit until cool enough to handle.
While sweet potatoes are cooking, make the chutney. Pulse cilantro, mint, jalapeno, ginger and garlic together in a food processor until well chopped. Mix together lime juice, water and olive oil in a bowl. Stream into the food processor while it's running, until it forms a creamy green spread. Season with salt to taste.
Scrape flesh of the sweet potatoes into a large bowl. Mix in chickpea flour, jalapeno, scallion, cilantro, spices, baking powder and salt until well combined.
Heat oil in a large skillet on medium-high heat. Drop golfball sized spoonfuls of falafel mix into the hot oil, pressing slightly with the back of a spatula to flatten and make a patty. Fry 3-4 minutes per side until golden and crispy, then remove to a plate. You'll have to do this in 3-4 batches.
Spread naan with a thin layer of yogurt, Top one side with lettuce and cucumber. Top vegetables with warm sweet potato falafels, drizzle with chutney and sprinkle with green onions. Serve.