• Wash sweet potatoes. Prick each 2 or 3 times with fork to allow steam to escape. Arrange in oven at least 1 inch apart. Microwave at HIGH (100%)until fork-tender, 8 to 10 mins. Cover and let stand 5 minutes.
• Peel and slice potatoes. Place in 2-quart casserole. Add 1/4 cup butter and the salt. Mash until no lumps remain. Mix in pineapple. Microwave at HIGH (100%) 2 minutes. Stir and set aside.
• Place 1 tablespoon brown sugar, 1 tablespoon butter, the water, cinnamon, and nutmeg in medium bowl. Microwave at HIGH (100%) until butter is melted, 1 to 1 1/2 minutes, stirring after half the time. Add marshmallows; toss to coat. Top sweet potato mixture with marshmallows.
• Microwave at HIGH (100 % until the marshmallows are melted and potatoes are heated through, 2 to 4 minutes. Sprinkle with pecans.