Method

• Prepare the pan: Heat the oven to 350°F (175°C). Lightly grease a 9-inch square metal pan and line it with parchment, leaving an overhang on two sides. Avoid a larger pan, which would make the base and fruit layer thinner.
• Make the oat crumble: In a bowl, mix the flour, oats, brown sugar, cinnamon, baking powder and salt. Pour in the slightly cooled melted butter and stir until clumps form. Reserve roughly one-third of the mixture for the topping.
• Set the base: Press the remaining mixture firmly and evenly into the pan. Bake for about 12 minutes, until the surface looks set. It does not need to be deeply browned.
• Mix the fruit filling: While the base bakes, toss the diced pears and cranberries with granulated sugar, cornstarch, lemon juice and vanilla until the fruit is evenly coated and no dry pockets remain. Do this shortly before filling the pan so the pears do not release a large puddle of juice.
• Add fruit and topping: Spread the fruit mixture and any small amount of juice evenly over the warm base. Scatter the reserved crumble in small clumps over the fruit.
• Bake until bubbling: Bake for 35–40 minutes, or longer if necessary, until the topping is golden, the filling bubbles near the center, and a fork slides into a pear piece easily. If the top darkens early, cover loosely with foil. Frozen cranberries can lengthen baking.
• Cool and slice: Rest the pan on a rack for about 30 minutes, then refrigerate to finish cooling and firm the filling, approximately 90 minutes more. Refrigerate within 2 hours of baking, or within 1 hour above 90°F (32°C). Lift with the parchment and cut into 12 bars only when completely cool.