Preheat the oven to 350 F and line a 9 x 13 inch pan with parchment or foil.
Place the sliced peaches in a bowl and toss gently with the lemon juice.
In a medium bowl, combine the cornstarch brown sugar, salt, cinnamon and nutmeg. Sprinkle on top of the peaches and stir gently to combine. Set aside.
In a large bowl, combine the oats, flour, brown sugar, baking powder, and salt. Stir in the melted butter.
Press 2/3 of the mixture evenly into the bottom of the pan, reserving the rest for the top.
Bake in the preheated oven for 10 minutes and let cool a bit.
With a spoon or small spatula, spread the raspberry jam evenly over the bottom crust. Add the peaches with all their juices on top and distribute evenly. Sprinkle evenly with the remaining crumble.
Return the pan to the oven and bake for an additional 30-40 minutes until the filling is bubbling (it must bubble to activate the cornstarch and allow the filling to set).
Cool completely (about 2 hours) and cut into bars.
*Canned or frozen peaches may be used. Canned peaches should be drained well and frozen should be thawed and patted dry.StorageStore in the refrigerator for 2-3 days.To freeze, wrap the bars individually with plastic wrap and place in an airtight freezer-safe container. Freeze for 3-4 months.