1. In a large pot, cover sweet potatoes with cold water. Add 1 Tbsp. salt. Bring to a boil over medium-high heat, then reduce heat and simmer until a knife can pierce sweet potatoes easily, 30 to 35 minutes. Drain potatoes, run under cold water to cool, then peel. Cut into 1/2-inch-thick slices. In a large bowl, toss pears with lemon juice.
2. Spread pecans on a baking sheet; bake until toasted and fragrant, shaking sheet once, about 7 minutes. Transfer pecans to a bowl; let cool.
3. Preheat oven to 350ºF. Grease a 9-by-13-inch baking dish. Layer half of sweet potato slices, overlapping, then top with half of pears, half of pecans and a pinch of salt. Drizzle half of butter on top. Repeat layers.
4. Cover baking dish tightly with foil and bake until sweet potatoes and pears are tender, about 1 hour and 15 minutes.
5. Preheat broiler. Sprinkle marshmallows over top of casserole and broil until marshmallows are lightly browned, about 1 minute.