Line a pie plate with crust and place in refrigerator, while you prepare the filling. Preheat oven to 400 degrees. Spread walnuts on a baking sheet and toast in oven 5 minutes, until there's a warm, nutty aroma. Cool, then chop coarsely and set aside.
Toss apple slices with lemon juice and cinnamon. Set aside.
Melt 1 tablespoon butter with honey and 1/2 cup brown sugar in a large heavy saudepan and heat, stirring constantly, until mixture comes to a rolling boil. Add apples, stirring to coat them with carmel. Reduce heat and cook uncovered no more than than 5 minutes. Do not overcook them to mushiness.
Place rhubarb in a bowl. Using a slotted spoon, transfer the hot apples into the bowl with the rhubarb. Add tapioca and stir to combine. Let sit for 15 minutes, add salt to the caramel remaining in the pan and cook stirring often to reduce to a thick syrup. Do not let it scorch. Remove from heat and set aside.
Combine streusel ingredients (flour, remaining 1/2 cup of brown sugar and 4 tablespoons cold butter), pinching the butter with your fingers until it's evenly distributed. Stir in the toasted walnuts.
Scrape the apple-rhubarb mixture into te chilled pie shell and drizzle with 3 tablespoons caramel. Spread streusel mixture over pie and bake at 400 for 30 minutes. Cool on a rack.
Just before serving may drizzle the remaining caramel (reheating if necessary) over the pie.
Calories 407 per serving
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