Place a rack in the upper third of the oven and preheat oven to 350 F. Generously spray a 7 x 11-inch or 9-inch square flat baking dish with nonstick spray.
For the streusel: In a large bowl, thoroughly stir together flour, oats, sugar, cinnamon, Rumford Baking Powder, and salt. Add the butter, stirring until the mixture is well blended and crumbly. Firmly press about half (no need to measure) the streusel into the baking dish, forming a packed, even layer. Stir the nuts (if using) into the remaining streusel; set aside for topping. Par-bake the crust in upper third of the oven for 12 minutes; the layer will not be browned.
For the filling: Meanwhile, combine the fresh and dried cranberries, jam, and sugar in a medium saucepan. Stirring constantly, bring to a full boil over medium-high heat. Boil, stirring, for 5 minutes. Remove from the heat. Using a table knife, evenly spread the cranberry mixture over the crust. Sprinkle the remaining streusel evenly over the filling. Pat down lightly.
Bake in the upper third of the oven for 25 to 35 minutes or until the top is lightly browned and the filling is bubbly. Transfer the pan to a wire rack. Let cool to warm. If desired, trim off and discard any overly brown edges all the way around using a large, sharp knife. Cut and serve the bars warm as individual desserts or coffee treats; or cut them small and serve at room temperature as cookies.