Heat your oven to 350°F. While it preheats, line an 8x8 inch baking dish (or 9x9 if you prefer thinner bars) with foil, leaving some overhang on the sides—this makes it easy to lift the bars out after baking and cooling.

In a large bowl, whisk together the flour, oats, brown sugar, baking soda, salt, and cinnamon. This ensures the leavening and spices distribute evenly throughout the oat mixture, preventing any pockets of baking soda flavor in the final bars.

Cut the cold butter into small cubes and add it to the dry ingredient mixture. Using a pastry cutter, fork, or your fingertips, work the butter into the flour mixture until it resembles coarse breadcrumbs with pea-sized pieces throughout. I find using cold butter straight from the fridge gives you the best texture—it creates little pockets that stay distinct rather than getting fully absorbed.

Firmly press 2 cups of the oat mixture into the prepared baking dish, creating an even, compact layer on the bottom. Press it down well so it holds together during baking. If the mixture feels too loose, it won't set properly as the base.

Spread the raspberry jam evenly over the base layer, staying about 1/4 inch from the edges to prevent overflow. Sprinkle the remaining oat mixture over the jam, then gently press it down with your hands or the bottom of a measuring cup. Press firmly but not aggressively—you want the top layer to hold together but not be rock-hard.

Bake for 35-40 minutes, until the top is light golden brown and the edges are slightly darker. The bars should feel set when you gently press them with a finger, though the center will still have a slight give. Don't overbake—oatmeal bars can dry out quickly if left in the oven too long.

Remove from the oven and let the bars cool completely in the pan on a wire rack—at least 30 minutes, though cooling for an hour gives cleaner cuts. Once cooled, use the foil overhang to lift the entire batch out, then cut into 12-16 bars depending on your preferred size. Store in an airtight container for up to 3 days.