Make the filling

• In a medium-sized saucepan, combine the diced rhubarb, strawberries, granulated sugar, lemon juice, cornstarch, and vanilla extract. Stir well to combine and ensure the sugar and cornstarch are evenly mixed.
• Place the saucepan over medium heat and bring the mixture to a simmer, stirring occasionally. As the mixture heats up, the fruits will release their juices and the mixture will start to thicken.
• Once the mixture reaches a simmer, reduce the heat to low and continue cooking for about 10-15 minutes, stirring occasionally. The rhubarb should become tender, and the mixture will thicken to a glossy consistency.
• Remove the saucepan from the heat and let the strawberry rhubarb filling cool completely. As it cools, it will continue to thicken. It will cool faster if you transfer it to a bowl.
• Grease a 13 x 9 x 2-inch metal pan with cooking spray or softened butter and line with parchment. Let the parchment paper hang over the long sides of the pan to make it easier to remove the bars for slicing when baked.
• To make the crust and topping, combine the flour, oats, salt, and sugar in a large bowl and whisk until incorporated.
• Grate the the cold butter and using a box grater or food processor with the shredding attachment, toss the butter into the flour mixture, rub gently if needed, until the butter is crumbly and pea-sized.
• Mix the yolk and vanilla together and, using a fork, stir them into the crumbly mixture. Switch to your fingers if you find that easier, and continue mixing the dough until the yolk and vanilla are fully incorporated and the dough holds together when you squeeze a bit between your fingers.
• Remove 330 grams of dough and place in a medium bowl in the refrigerator. Press the remaining dough firmly and uniformly into the prepared pan.
• Evenly spread the cooled filling over the bottom crust.
• Sprinkle the refrigerated dough over the jam, breaking it into little balls between your fingers for the ultimate crumble topping.
• Preheat the oven to 350 F.
• Bake the bars for about 45 minutes, rotating the pan at the halfway mark, until the oat crumb topping is lightly browned.
• Let cool to room temperature before slicing and serving. Use the parchment paper to help lift the bars out of the pan to slice.