1. Make topping: In a bowl, mix flour, oats, sugar, salt and zest. Rub in butter until mixture resembles bread crumbs. Freeze.
2. Make filling: Mix berries, sugar and zest in a heavy pan over medium heat. Cook, stirring, until sugar dissolves and fruit softens, 8 to 10 minutes.
3. Remove topping from freezer; crumble with your fingers. Cook in a heavy skillet over medium-low heat, stirring and tossing until it turns golden, about 10 minutes. Watch carefully and toss well to keep it from burning. Spread topping on a piece of parchment; let cool for 3 minutes. Mixture will firm up as it cools.
4. Scoop ice cream into 4 bowls. Spoon berry mixture and topping over. Serve immediately.