For topping: Mix first 6 ingredients in medium bowl. Add melted butter and vanilla extract; stir with fork until soft crumbly dough forms. (Can be made up to 6 hours ahead. Cover; let stand at room temperature.)
For custard: Whisk eggs in medium bowl to blend. Whisk in crème fraîche and vanilla. Sift in flour; whisk until smooth. (Can be made up to 6 hours ahead. Cover and chill; rewhisk before using.)
For filling: Preheat oven to 375°F. Generously butter 13x9x2-inch oval ceramic or glass baking dish. Combine pears, cherries, and lemon juice in large bowl; toss to blend. Mix flour, sugar, and lemon peel in small bowl. Add to pear mixture and toss to blend. Transfer pear mixture to prepared dish, pressing slightly with rubber spatula to form even layer. (Can be made up to 1 hour ahead; let stand at room temperature.)
Pour custard evenly over pear mixture. Using fingertips, crumble topping evenly over. Bake until pears are tender and topping is deep golden, about 55 minutes. Remove from oven and let stand 10 minutes. Serve warm or at room temperature, with ice cream.