Rhubarb filling (makes about 1½ cups)

Cup of Yum
1. Place the rhubarb, sugar, apple, lemon juice, water, and salt in a saucepan (I use a 5 qt / 1 L pot for 1 batch). Mix well to combine.
2. Place the pot over medium-high heat and cook until the mix comes to a boil. Stir frequently while cooking. This should take about 10 minutes.
3. Reduce the heat to medium / medium-low and continue to cook the mixture while stirring frequently. The mixture may start to bubble, so be careful!
4. Cook until the mixture starts to thicken and it looks like apple butter. This can take a further 20 - 30 minutes.
5. Once thickened, remove from the heat and transfer the filling into a bowl. Let it cool down to room temperature. Cover and let it chill in the fridge until the next day if needed, but the filling can be used at room temperature (or slightly warmer as well).

Shortbread dough

1. Make sure the butter is soft and at room temperature. You can use cold butter as well, but it will be harder to mix. If the butter is hard and not easily pressed, place the butter in a warm area until it’s soft (or microwave on low power). Do not let it melt.
2. In a large bowl, mix the flour, sugar and salt.
3. Add the butter, egg, and vanilla.
4. Using a spatula, mix the butter, egg and flour mix together. You can also use your hands to mix the dough.
5. Make sure the butter and egg are mixed through the flour uniformly and forms a soft dough.
6. Divide the dough into two pieces - ¾ of the dough and a smaller ¼ of the dough. Cover the dough and let it rest while you prepare the baking pan.

Assembly and baking

1. Preheat the oven to 350°F / 180°C.
2. Butter and line a 9 inch square pan with parchment paper.
3. Take the larger portion of shortbread dough (¾ portion), and press it into the bottom of the baking pan. Make sure the dough is pressed into a flat, even layer. Use a flat surface like a spatula or the bottom of a glass to evenly press the dough into the base.
4. Spread about 1 cup of the cooled rhubarb filling over the shortbread layer.
5. OPTIONAL - add the almond or oats to the ¼ portion of the dough. Break up the dough into smaller pieces and mix it with the nuts / oats.
6. Take smaller pieces of the dough and flatten them slightly. Place these flattened dough pieces on top of the rhubarb filling layer. You can place enough to cover the whole rhubarb filling or leave some gaps to have the filling peaking through.
7. Transfer the baking pan into the preheated oven and bake for about 40 minutes, rotating the pan once halfway through the baking time. Remove the pan from the oven when there is some browning on the surface.
8. Let the bars cool down to room temperature. Preferably let it chill for a few hours in the fridge to make it easier to cut them into bars.
9. Remove the rhubarb bars from the pan and cut into squares or bars. I cut them into 16 squares.
10. Store the bars in an airtight container for up to 1 week in the fridge.