1. Make streusel: Preheat oven to 350°. Line a rimmed baking sheet with parchment. In a bowl of a stand mixer, mix dry ingredients. Add butter; mix on low speed until crumbly. Spread in pan. Bake until golden at edges, 8 to 10 minutes; stir. Bake until golden all over, 3 to 5 minutes. Let streusel cool. Increase oven to 375°.
2. Make pears: Peel and core fruit, cut into 1-in. chunks, and mix gently in a large bowl with lemon juice. Butter a shallow 2-qt. baking dish and set aside.
3. Combine sugar and 2 tbsp. water in a 3- to 4-qt. pan. Cook over medium-high heat until sugar begins to brown in 1 spot, 4 to 7 minutes. Swirling pan occasionally, boil until sugar is golden brown, 1 to 2 minutes more. Remove from heat, let cool about 30 seconds, then gently stir in pears (sugar will seize up in spots). Return pan to heat and cook, stirring occasionally, until sugar melts again, 2 to 3 minutes.
4. Add marsala, ginger, raisins, vanilla, and salt. Reduce heat and simmer, stirring occasionally, until pears are just tender, 8 to 10 minutes. Using a slotted spoon, transfer pears to prepared dish. Add butter to pan and cook until melted. Spoon mixture over pears.
5. Scatter streusel over pears, then bake until fruit is bubbling, 15 to 20 minutes. Let cool about 10 minutes. Serve warm, with vanilla ice cream or creme fraîche.
Make ahead: Up to 3 hours, then reheat in a 350° oven until warm, about 20 minutes.
Note: Nutritional analysis is per serving.