Preheat Oven:Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
Make Crust & Crumble:In a large bowl, mix flour, oats, brown sugar, salt, and cinnamon. Stir in melted butter and vanilla extract until crumbly and well combined.
Press Crust:Reserve 3/4 cup of the mixture for the topping. Press the remaining mixture firmly into the bottom of the prepared pan to form the crust.
Add Strawberry Filling:Spread strawberry jam evenly over the crust. If using, scatter diced fresh strawberries over the jam. Drizzle with lemon juice.
Add Crumble Topping:Sprinkle the reserved oat mixture evenly over the strawberry layer.
Bake:Bake for 30-35 minutes, or until the top is golden and the filling is bubbly.
Cool & Slice:Let bars cool completely in the pan before lifting out and slicing into squares.