Step 1: Macerate the Rhubarb. Begin by tossing the diced rhubarb with granulated sugar in a medium bowl. Let it sit for about 1 hour to allow the sugar to draw out the moisture while softening the rhubarb. After an hour, drain any excess liquid, pat the rhubarb dry using paper towels, and set it aside as you prepare the jam.

Step 2: Prepare the Strawberry-Rhubarb Jam. In a medium saucepan over medium heat, combine the quartered strawberries, diced rhubarb, sugar, kosher salt, and lemon juice. Cook the mixture for approximately 12-15 minutes, frequently stirring until it thickens and bubbles. You’ll know it’s ready when it coats the back of a spoon. Remove it from heat and proceed to make a cornstarch slurry for adding to the jam.

Step 3: Thicken the Jam. In a small bowl, stir together the cornstarch and cold water until smooth to create a slurry. Slowly add this mixture to the cooked jam, stirring continuously for an additional 2 minutes until the jam becomes glossy and thick. Once done, transfer the jam to a bowl and allow it to chill in the refrigerator as you prepare the cookie dough.

Step 4: Make the Cookie Dough. In a large mixing bowl, whisk together the melted and cooled butter, light brown sugar, and granulated sugar until smooth. Add the lemon zest for a refreshing aroma, then incorporate the egg yolks and vanilla extract. Mix until everything is well combined, creating a rich base for your Strawberry Rhubarb Oat Crisp Cookies.

Step 5: Combine Dry Ingredients. Sift the all-purpose flour, rolled oats, baking powder, baking soda, and kosher salt into the wet mixture. Gently fold these dry ingredients into the wet ingredients until just combined. Be careful not to over-mix, as you want the dough to maintain a light and fluffy texture for your cookies.

Step 6: Incorporate Rhubarb and Jam. Gently fold in the macerated rhubarb, being careful to maintain the chunks. Next, spoon in dollops of the chilled strawberry-rhubarb jam without fully combining it into the dough; you want pockets of jam in each cookie.

Step 7: Chill the Dough. Line a baking tray with parchment paper and scoop spoonfuls of the cookie dough onto the tray, spaced a few inches apart. Cover the tray with plastic wrap and chill the dough in the refrigerator for about 1 hour.

Step 8: Bake the Cookies. Preheat your oven to 350°F (175°C). Once heated, bake the cookies for 20-22 minutes or until the edges are golden brown and the tops are set but still slightly soft.

Step 9: Cool the Cookies. After baking, let the cookies cool on the baking sheet for 15-20 minutes. Once they’re slightly cooler, move them to a wire rack to cool completely for about 1 hour.

Step 10: Optional Glaze. If you want to add a special touch, prepare the optional glaze by mixing the reserved rhubarb syrup with milk and powdered sugar until it reaches your desired consistency. Drizzle this glaze over the cooled cookies for an extra layer of sweetness and flavor.