Line an 8 x 8-inch baking pan with foil so that it extends up the sides. Line the bottom of the pan with a square of parchment paper.
To make the crust and topping, combine the pecans, oats, salt, and cinnamon in a food processor and pulse until coarsely ground. Add the prunes and pulse until well combined. Add the coconut oil and pulse until the mixture looks clumpy and moist. Pinch some between your fingers--it should feel tacky, not sandy. If it is too dry, add an additional tablespoon of coconut oil. Transfer slightly more than half the mixture to the prepared pan, reserving the rest. With clean hands, firmly and evenly press it into the bottom of the pan.
To make the filling, combine the apples, brown sugar, and cinnamon in a medium saucepan. In a small bowl, whisk the cornstarch and water to blend. Pour over the apples. Bring to a boil, then reduce the heat and simmer until the mixture is thick and the apples are soft but still hold their shape, 15 to 20 minutes.
Spread the apple filling evenly over the crust in the pan. Crumble remaining pecan mixture over the apples and press it in gently. Transfer the pan to the refrigerator and chill until very firm, at least 4 hours.
Using the foil as handles, lift the whole piece out of the pan and cut into bars. Store bars in the refrigerator.