For the filling: Peel and core the apples, and cut them into 1/2-inch-thick slices. Toss them with the granulated sugar and lemon juice. Melt the butter in a large skillet over medium-high heat. Add the apples, and cook, stirring occasionally, until the firmer apples soften but still hold their shape, 15 to 18 minutes. Add the cream, flour, cinnamon and salt, and stir until the juices thicken, about 2 minutes. Remove the skillet from the heat, and let the filling cool completely. Transfer it to a 9-by-13-inch baking dish.
For the topping: Mix the flour, oats, walnuts, granulated sugar, butter and salt together in a medium bowl, using your fingers to work the butter into the flour, until the mixture comes together and holds its shape when squeezed.
Drop small clumps of the topping over the filling, and wrap the baking dish well with plastic wrap. Label and date, and freeze for up to 2 weeks.
To serve: Position an oven rack in the top third of the oven, and preheat to 350 degrees F. Unwrap the crisp, and bake, frozen and uncovered, until the filling is hot and bubbly and the top is golden brown, 50 minutes to 1 hour. Dust the crisp with confectioners' sugar, and serve warm with scoops of vanilla ice cream if you like.
To eat the crisp right away instead of freezing, after adding the topping, bake the crisp until the filling is hot and bubbly and the top is golden brown, 35 to 40 minutes.
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