Preheat oven to 350 degrees. In a food processor, pulse bread until very coarse crumbs form (you should have about 3 cups). Add 2 tablespoons oil; pulse briefly to moisten. Season with salt and pepper; set breadcrumbs aside.
In a medium saucepan, heat remaining 2 tablespoons oil over medium-high heat. Add sage; cook until crisp, 2 to 3 minutes. Using a slotted spoon, transfer sage to a paper-towel-lined plate; set aside (reserve pan with oil).
Add onion and garlic to pan; cook, stirring occasionally, until onion is tender, 5 to 7 minutes. Add sausage and wine; cook, breaking sausage up with a wooden spoon, until cooked through, 3 to 5 minutes. Stir in beans; cook until beans are tender and creamy, about 5 minutes. Season with salt and pepper.
Transfer sausage-and-bean mixture to a shallow 4-quart baking dish; scatter breadcrumbs over top. Place dish on a rimmed baking sheet, and bake until topping is golden, 25 to 30 minutes. Let cool 5 minutes before serving. Serve casserole topped with fried sage leaves.