1. Prepare Oven and Pan:

• Preheat oven to 350°F (175°C).
• Line an 8-inch square baking pan with aluminum foil or parchment paper, leaving an overhang on two opposite sides for easy lifting later. Spray the lining lightly with cooking spray. Set aside.

2. Make the Crust and Crumble Topping Mixture:

• In a large, microwave-safe bowl, melt the ½ cup of unsalted butter (about 1 minute on high, or use a saucepan). Let cool slightly.
• Add the all-purpose flour, old-fashioned oats, granulated sugar, packed light brown sugar, and salt to the bowl with the melted butter.
• Stir with a fork or spatula until combined. The mixture will look dry and sandy, with some larger clumps – this is correct.
• Measure out and set aside 1 heaping cup of this mixture to use later as the crumble topping.

3. Press the Crust Base:

• Transfer the remaining crumble mixture (everything except the reserved 1 cup) to the prepared lined pan.
• Using a spatula, the bottom of a measuring cup, or your clean fingers, press this mixture firmly and evenly across the bottom of the pan to create a flat, packed crust layer. Set aside.

4. Make the Strawberry Filling:

• In a medium saucepan, combine the coarsely chopped fresh strawberries, ⅓ cup granulated sugar, fresh lemon juice, and cornstarch.
• Stir everything together well.
• Place the saucepan over medium heat. Bring the mixture to a gentle simmer, stirring frequently.
• Continue to simmer, stirring often, for about 5-8 minutes, or until the strawberries have softened and the juices have thickened into a jammy consistency.

5. Assemble the Bars:

• Pour the hot strawberry filling evenly over the prepared crust base in the pan. Spread it out gently with a spatula.
• Sprinkle the reserved 1 heaping cup of the crumble mixture evenly over the top of the strawberry layer.

6. Bake:

• Place the pan in the preheated oven.
• Bake for 30-35 minutes, or until the crumble topping is lightly golden brown and the strawberry filling is bubbly around the edges.

7. Cool Completely:

• Remove the pan from the oven and place it on a wire cooling rack.
• Let the bars cool completely in the pan before attempting to slice them. This is crucial for them to set properly and cut cleanly. Cooling for at least 2-3 hours at room temperature, or speeding up the final cooling in the fridge, is recommended.

8. Slice and Serve:

• Once completely cool, use the foil or parchment overhangs to lift the entire block of bars out of the pan onto a cutting board.
• Use a sharp knife to cut into squares or rectangles.
• Serve at room temperature. Store leftovers in an airtight container at room temperature for a couple of days, or refrigerate for longer storage.