Preheat oven to 375°F. On a baking sheet toast almonds until golden, 10 to 15 minutes. Cut butter into bits. In a food processor blend flour, brown sugar, 1/2 cup oats, cinnamon, salt, and butter until mixture resembles coarse meal. In a bowl stir together flour mixture, remaining 1/4 cup oats, and almonds. In a small bowl stir water and cornstarch until combined. Halve and pit plums. In a 12-inch skillet cook plums and sugar over moderate heat, stirring, until sugar is melted. Stir in cornstarch mixture and simmer, stirring, 15 minutes, or until mixture is thickened. Transfer plum mixture to a 15 x 9-inch (3-quart) baking dish. Sprinkle topping over plums. Bake mixture in middle of oven 40 to 45 minutes, or until topping is crisp and golden, and cool on a rack 10 minutes. Serve dessert warm with ice cream.