Start by washing and slicing the rhubarb into small pieces, then place it in a medium saucepan along with fresh raspberries, fine granulated sugar, and tapioca or cornstarch. Cook over medium heat for about 20-30 minutes, stirring frequently until the mixture thickens and bubbles, mashing the fruit gently as it cooks.

In a frying pan, add the old-fashioned rolled oats and toast them over medium heat for about 5 minutes, stirring regularly to prevent burning.

While the oats are toasting, preheat your oven to 350°F (175°C). Prepare a 9x9 inch baking pan by greasing it and lining it with parchment paper.

In a large mixing bowl, cream together the softened unsalted butter, fine granulated sugar, salt, baking soda, ground cinnamon (if using), and beaten egg until the mixture is light and fluffy.

Take two-thirds of the oat mixture and press it firmly into the bottom of the prepared baking pan. Spread the cooled rhubarb and berry jam over this base.

Crumble the remaining dough over the jam layer, sprinkling it evenly to create a topping for your Oatmeal Rhubarb Bars.

Place the assembled bars in your preheated oven and bake for 25-30 minutes until the top is golden brown and crispy.