Getting ready:

1. Preheat your oven to 350 degrees F.
2. Line the bottom and up the sides of a 9 x 9 inch baking pan with parchment paper. Set aside.

Making the Rhubarb Filling:

1. In a saucepan add the chopped rhubarb, sugar, water and lemon juice.
2. Simmer on low-medium heat for about 10 minutes. You want the rhubarb soft, but still chunky.
3. Set aside.

Making the Oat Bar:

1. In a large bowl, mix the flour, oats, sugar and salt together.
2. Stir in the melted butter. Mix until combined.
3. Measure out 1/2 cup of the oat mixture and set it aside for later. 
4. Add the remaining oat mixture to your prepared baking pan. Press the mixture firmly and evenly into the bottom the pan.

Putting it all together:

1. Spread the cooked rhubarb mixture evenly over the oat crust.
2. Sprinkle the 1/2 cup of oat mixture you had set aside, over the top of the rhubarb. You can sprinkle it evenly or randomly over the top.
3. Sprinkle the top with cinnamon and sugar.
4. Bake for 1 hour, until the top is nice and golden and the rhubarb is bubbling.
5. Let cool for at least 2 hours before cutting!