Peel and core apples and pears. Cut fruit lengthwise into about 1/2-inch-thick wedges; place in a large bowl.
Grate peel (yellow part only) from lemon, then ream juice. Add peel and juice to apples and pears, along with sugar, currants, and rum; mix gently. Pour into a shallow 2- to 2 1/2-quart baking dish.
Place biscotti in a 1-quart zip-lock plastic bag and crush lightly with a rolling pin to make 1 1/2 cups coarse crumbs; pour into a small bowl. Add butter and cinnamon and mix. Sprinkle crumb mixture evenly over fruit. Pour cream evenly over topping.
Bake in a 375° regular or convection oven until fruit is tender when pierced and crumb topping is browned, 30 to 40 minutes. Let gratin cool at least 10 minutes; serve warm or cool. Scoop onto plates or bowls and top with ice cream.