Begin by preheating the oven to 350F. Line an 8-inch square pan with aluminum foil and coat it with cooking spray; this will facilitate easier cleanup and is highly recommended.

For the crust and crumble topping, melt the butter in a large microwave-safe bowl for about 1 minute on high power. Add the sugars and whisk until combined. Mix in the flour and optional salt until a dry, sandy mixture forms with some larger crumble pieces. Reserve a heaping 3/4 cup of this mixture.

Press the remaining mixture into the prepared pan to form an even, smooth crust using a spatula or your fingers. Set aside.

For the filling, in a medium bowl, combine the egg, Greek yogurt, sugar, lemon juice, and vanilla. Whisk until smooth, then add the flour and mix until well combined. Pour the filling evenly over the crust, smoothing it out with a spatula. Set aside.

Prepare the strawberry layer by mixing all the ingredients (except flour) in a separate medium bowl. If the mixture is very thin, add up to 2 tablespoons of flour to thicken it. Distribute the strawberry mixture evenly over the filling, smoothing it lightly with a spatula.

Sprinkle the reserved 3/4 cup crust mixture evenly over the strawberry layer, squeezing it in your palm to create larger crumble pieces before sprinkling.

Bake for 60 to 75 minutes, placing the pan on a cookie sheet to catch any overflow. The edges should bubble vigorously, and the crumble topping should appear set and pale golden.

Allow the bars to cool in the pan on a wire rack for at least 2 hours (preferably overnight) before slicing. For cleaner cuts, chill in the fridge for 2 hours before slicing. Store the bars airtight at room temperature for up to 5 days, in the fridge for 7-10 days, or freeze for up to 4 months.