Add the salted butter to a small saucepan, over medium heat. Melt the butter, and continue to cook, whisking constantly, until it smells nutty, and begins to brown, about 6-7 minutes. Pour into a heatproof bowl, and set aside to cool slightly.

Make the filling: Add the fresh raspberries, raspberry preserves, and lemon juice to a small sauce pan, and cook on medium heat until the raspberries have mostly broken down. Use the back of a spoon to gently smash the strawberries. This should take about 10 minutes. Set aside to cool.

OPTIONAL: To make your own raspberry jam: place 2 cups of fresh raspberries in a small saucepan. Add 1/2 cup granulated sugar. Cook on medium heat, stirring occasionally to breakdown the berries, and prevent the berries from burning on the bottom of the pan. Once the mixture has liquified, simmer over medium heat for about 20-25 minutes. The jam will reduce as it simmers, and it is ready when it has reduced by at least 1/2. Pour the jam into a heatproof and allow it cool completely.

Preheat the oven to 350 degrees F. In a medium bowl, add the browned butter, brown sugar, vanilla extract, kosher salt, 1 cup of the oats, and all-purpose flour. Use a spoon to mix, until dough comes together, is crumbly, and there are no dry patches of flour.

Line an 8×8″ baking pan with parchment paper. Pour 1/2 of the mixture into the prepared baking pan. Use the bottom of a cup or measuring cup to press the mixture down into an even layer. Spread the raspberry filling over the bottom crust.

Add the remaining 1/2 cup of oats to the remaining dough, and evenly sprinkle the crumbly mixture on top of the raspberry filling. Bake for 50-60 minutes, until lightly golden brown. Allow to cool completely, then slice into 9 large square bars, or 18 smaller bars.