1. Preheat oven to 400°. Make crumble: Combine granulated sugar and cornstarch in a large bowl. Stir in lemon zest and juice and 1/3 cup water. Cut unpeeled fruit into wedges and large chunks, catching juices over a bowl, and gently mix fruit and juices into sugar mixture. Transfer to 2 cast-iron skillets (9 to 10 in. each) or a 9- by 13-in. baking dish (use metal if you plan to reheat on a grill).
2. Pulse brown sugar, flour, and salt in a food processor to blend. Add oats, almonds, butter, and 2 tbsp. water and pulse just until evenly moistened. Scatter over fruit.
3. Bake until topping is browned and juices are bubbling, 40 to 45 minutes. Let cool on racks at least 15 minutes.
4. Make whipped ricotta: Beat cream and vanilla seeds in a bowl with a mixer until stiff peaks form. Add the ricotta, lemon zest and juice, and honey to taste and beat until well combined. Top crumble with spoonfuls of ricotta and garnish with mint.
*Split the bean lengthwise, then scrape out the seeds with the tip of a small knife.
Make ahead: For crumble and ricotta, up to 1 day, chilled. Let crumble come to room temperature, or serve warm. The OITF team reheats the crumble on a grill, tented with foil; you could also bake it just before your outing and transport it warm on a thick towel.