1. Preheat the Oven and Prepare the Pan

• Preheat your oven to 350°F (175°C).
• Line an 8×8-inch baking pan with parchment paper or foil. Leave an overhang for easy lifting later.
• Lightly grease the lining with butter or cooking spray.

2. Make the Oatmeal Crust and Crumble

• In a large mixing bowl, combine the following:
  • Melted butter
  • All-purpose flour
  • Rolled oats
  • Granulated sugar
  • Brown sugar
  • Salt
• Mix with a spoon or spatula until the mixture becomes crumbly and resembles wet sand with some clumps.
• Set aside 1 cup of the mixture to use as the crumble topping later.

3. Press the Crust Layer

• Pour the remaining oat mixture into the prepared pan.
• Press it down evenly and firmly using your fingers or the flat bottom of a measuring cup.
• Make sure the crust reaches the edges and is well-compacted.

4. Prepare the Mango Filling

• In a medium bowl, toss together the chopped mango with:
  • Granulated sugar
  • Lemon juice
  • Cornstarch
• Stir gently until the fruit is evenly coated and the cornstarch is dissolved.
• The lemon juice brightens the flavor and helps balance the sweetness.

5. Add the Mango Layer

• Spread the mango filling evenly over the crust layer.
• Make sure to distribute the chunks well so that each bite has mango flavor.

6. Add the Crumble Topping

• Sprinkle the reserved 1 cup of oat mixture over the mango layer.
• Distribute it evenly but don’t press it down—this layer should stay crumbly.
• Allow small gaps where the mango can peek through.

7. Bake the Bars

• Place the pan in the oven and bake for 28 to 32 minutes, or until:
  • The top crumble is golden brown
  • The mango filling is bubbling slightly at the edges
• Remove the pan from the oven and let it cool on a wire rack for at least 1 hour.

8. Cool and Slice

• For best results, refrigerate the pan for another 30–60 minutes before slicing.
• Use the parchment overhang to lift the bars out of the pan.
• Slice into 9 squares (or 12 smaller ones, depending on your preference).