My husband, who is a type 1 diabetic, loves strawberry rhubarb pie. To reduce the carbs, I'd like to do a crisp, rather than pie. Also I have about 1 cup of strawberry purée that I froze a few weeks ago that I'd like to use up (just strawberries blended with 1 package of Splenda and perhaps a small amount of water). So I am thinking of mixing some rhubarb with the strawberry purée, adding some lemon juice and zest, and a bit of flour and a bit more sweetener, and then topping with a crumble.
Questions: how much rhubarb should I add, given this amount of purée? Should I add some sliced berries too? If so, how much? Any other flavorings, such as cinnamon or nutmeg? How much sweetener - you can express it in terms of sugar and I'll substitute Splenda (and please don't suggest stevia, agave syrup, etc instead. Splenda may taste off to you but he is fine with it and it's what I have on hand.). He doesn't have much of a sweet tooth, so I want it to be pretty tart, so bear that in mind in terms of how much sweetener you suggest.
Also how should the rhubarb be prepared -- i.e., what size pieces? Does it need to be cooked before it is added to the filling?
As to the crumble, I pretty much know what I will do as I often make apple or other fruit crisps - whole wheat flour, rolled oats, chopped nuts, butter, and a bit of sweetener. Should I add some cinnamon to the crumble. I always do with apple crisp but not sure whether it would complement the strawberry, rhubarb?