Make the Macerated Rhubarb: In a small jar or bowl, toss the rhubarb with the sugar until evenly coated. Let sit for 1 hour, then drain thoroughly and pat very dry with paper towels.

Make the Strawberry Rhubarb Jam: In a small saucepan, combine the strawberries, rhubarb, sugar, salt, and lemon juice. Cook over medium heat for 12–15 minutes, stirring frequently until the mixture becomes very thick and jammy. In a small bowl, whisk together the cornstarch and cold water. Stir the slurry into the jam and cook for 2 minutes more, until glossy and thickened. Remove from the heat and stir in the vanilla extract. Transfer the jam to a freezer-safe bowl or shallow container and freeze for 20–30 minutes, stirring once halfway through, until cold.

Chill & Bake the Cookies: In a large mixing bowl, whisk together the melted butter, brown sugar, granulated sugar, and lemon zest until combined. Add the egg yolks and vanilla extract and whisk until glossy and smooth. Add the flour, oats, baking powder, baking soda, salt, and strained macerated rhubarb. Gently toss the rhubarb with the flour mixture before you fold to help prevent soggy pockets in the dough. Fold until no dry streaks of flour remain. The dough will be very thick. Add about half of the chilled jam in small spoonfuls across the surface of the dough. Using a spatula, gently fold the dough over the jam just until the spoonfuls are mostly covered. Do not fully mix. Dollop the remaining jam across the surface of the dough. Do not fold again. Using a large cookie scoop, scoop through the jam-topped surface, making sure each scoop catches some jam on top while the bottom of each dough ball is mostly plain cookie dough. Place the cookie dough balls on a parchment lined sheet tray. Chill the cookie dough balls in the fridge for 1 hour. Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Arrange the chilled cookies several inches apart to allow for spreading. Bake for 20–22 minutes (smaller cookies will bake faster), until the edges are golden brown and the tops look matte and no longer wet. Let cool on the baking sheet for 15–20 minutes before transferring to a wire rack. For the best texture, let the cookies cool for about 1 hour before serving to allow the centers to fully set.