In a large bowl, combine the rhubarb, strawberries, 3/4 cup of the granulated sugar, orange zest, cornstarch and orange liqueur, and toss to thoroughly combine. Grease a 9 by 13-inch casserole or baking dish with 1 tablespoon of the butter and transfer the fruit mixture to the prepared casserole.
Preheat the oven to 375 degrees F.
In a mixing bowl, combine the remaining 9 tablespoons of butter, flour, remaining 1/4 cup of granulated sugar, the brown sugar and salt and cut together until the mixture resembles coarse crumbs. Sprinkle the topping evenly over the fruit and bake until the topping is golden brown and crispy and fruit is bubbly in places, about 45 minutes.
Cool briefly and serve warm, with a dollop of whipped cream, ice cream, or creme fraiche, if desired.
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