Preheat your oven to 350°F (175°C).

In a large bowl, combine the flour, rolled oats, brown sugar, and salt. Mix well before adding the softened butter.

Work the butter into the dry ingredients until you achieve a crumbly texture. Reserve about 1/2 cup of this mixture for the topping.

Grease an 8x8-inch baking dish. Press the remaining crust mixture firmly into the bottom of the pan to form a solid base.

Bake in the preheated oven for about 10 minutes, until lightly golden.

While the crust is baking, toss the fresh raspberries in a bowl with lemon juice and granulated sugar.

Gently mix to coat the berries.

Remove the crust from the oven and evenly spread the raspberry mixture over the slightly cooled base.

Sprinkle the reserved crumble mixture over the top.

Return the pan to the oven and bake for an additional 15-20 minutes, or until the topping is golden brown and the raspberries are bubbling.

Let the bars cool in the pan for at least 30 minutes before cutting into squares.