Preheat the oven to 375 degrees F. Place a rack in the middle of the oven and another in the lower third. Combine the oats, walnuts, sugar, salt and mashed banana, then cut in the butter. Spread the mixture on a parchment-paper-lined baking sheet. With damp hands, pat the mixture into an even 1/4-inch layer. Place the sweet potatoes, cut-side up, in a baking pan and place on the bottom oven rack. Place the topping on the upper rack. Bake the topping until the edges are lightly browned, 15 to 18 minutes. Cool the topping, but leave the potatoes in the oven to continue cooking until a knife inserted in the center comes out easily, about 1 hour total. When the topping is cool enough to handle, break into pea-size crumbles. Use a fork to scrape and fluff the insides of the potatoes. Sprinkle the potatoes with the crumb topping, then drizzle each with some of the honey and serve immediately.
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