Pour the melted, slightly cooled butter over the oat-flour mixture. Stir with a spoon or spatula until the mixture looks evenly moistened and crumbly, with no dry flour pockets. It should clump together when you squeeze a bit in your hand.
Create the first layer (the base crust): Scoop about 2/3 of the buttery oat mixture into the prepared baking dish. Press it firmly and evenly into the bottom using clean hands or the flat bottom of a measuring cup. This forms a sturdy, buttery crust that will hold the bars together.
Create the second layer (the strawberry filling): Spoon the strawberry preserves over the pressed crust. Use an offset spatula or the back of a spoon to gently spread it into an even layer, all the way to the edges and corners. Try not to dig into the crust as you spread.
Create the third layer (the crumble topping): Sprinkle the remaining 1/3 of the oat mixture evenly over the strawberry layer. Don’t press it down too firmly; you want loose, craggy bits so the jam can peek through as it bakes and you get that pretty, golden, crumbly top.
Create the fourth layer (the golden baked finish): Place the dish on the middle rack of your preheated oven and bake for 25–30 minutes, or until the top is lightly golden and you see the strawberry layer bubbling a bit around the edges. The bars will still look a little soft in the center but will firm up as they cool.
Cool the bars completely in the glass dish on a wire rack, at least 1–2 hours, so they set up and slice cleanly. Once cooled, cut into 12 squares or smaller bite-size pieces. Serve straight from the clear glass baking dish so the layers are visible and inviting.
