Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper. Set aside

In a large bowl using a hand or stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, and granulated sugar on medium speed for 2–3 minutes until light and fluffy.

With a rubber spatula, scrape down the sides and bottom of the bowl. Add the egg and beat on medium-low speed until fully incorporated and smooth.

All at once, add the the flour, cornstarch, oats, cinnamon, nutmeg, baking powder, and salt. Mixing on low speed until just combined. Do not overmix.

Use a 3-tablespoon cookie scoop to portion dough. Press two scoops together to form one large cookie, then roll into a ball with your hands.

Place 3–4 dough balls per sheet, spaced 3 inches apart. Bake for 14–15 minutes or until the edges are golden and the centers still look soft.

While still warm, gently press each cookie down with a flat spatula or the bottom of a glass until about ½ inch thick.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

In a medium saucepan, combine all berries, sugar, lemon juice, lemon zest (if using), and a pinch of salt.

Lightly mash the berries with a potato masher or spoon to release some juice.

Bring to a simmer over medium heat, stirring frequently to avoid burning.

Simmer for 20–25 minutes until thickened and jammy, stirring occasionally.

If desired, blend briefly with an immersion blender for a smoother texture or strain to reduce seeds.

Let cool slightly. Transfer to a container and refrigerate until ready to use (up to 2 weeks).

Preheat oven to 400°F (205°C). Line a small baking sheet with parchment paper.

In a medium bowl, mix the brown sugar, flour, oats, cinnamon, and salt. Cut in the cold butter using a fork or your fingers until the mixture forms small clumps.

Spread evenly on the prepared sheet and bake for 8–10 minutes, stirring halfway through, until lightly golden.

Cool completely, then break apart any large chunks.

In a mixing bowl, beat the softened cream cheese and butter together on medium speed for 2–3 minutes until creamy.

Scrape down the sides. Add the powdered sugar gradually on low speed.

Add vanilla, cinnamon, and salt. Increase speed to medium-high and beat for another 2–3 minutes until smooth and fluffy.

Once cookies are completely cooled, spoon 1–2 tablespoons of the triple berry jam onto the center of each cookie. Spread evenly, leaving a slight border.

Fill a piping bag (or ziplock with a snipped corner) with the cinnamon cream cheese frosting. Pipe a swirl or dollop on top of the jam.

Sprinkle generously with the cinnamon streusel.

Serve immediately or chill for 10–15 minutes for a firmer texture.