I made the pie as a crumble because I generally find crumbles both easier and tastier than single crust fruit pies. I realized belatedly that 2 cups of rhubarb was an absurdly small amount, and rifled the freezer for some plums to bulk up the fruit. The plums and rhubarb turned out to be a congenial combination. I suppose the only reason it hasn't garnered more fame is that plums and rhubarb aren't really co-seasonal.
When I make a crumble topping, I usually do it entirely by feel. This time I thought maybe I could make up a simple rule, so I tried using a half cup of everything, except the butter, of which I used a half stick. This made a serviceable topping, but nothing like what it could be. I have suggested a variation in the recipe below that should be closer to the mark.
Rhubarb Cream CrumbleNote: With only two cups of fruit, this dish is more like a clafouti. With the addition of plums, the "cream" becomes just a tasty thickener.
Place 2 cups diced rhubarb and 3/4 pound halved Italian plums in a 9 inch square casserole.
2/3 cup sugar
2 Tbsp flour
2 egg yolks
2 tsp lime juice (or lemon juice or water)
1/8 tsp salt
The consistency will be similar to creamed butter and sugar.
Spread mixture over fruit and bake at 400 degrees F for 20 minutes.
While fruit is baking, make crumble topping.
1/2 cup packed brown sugar
1/2 cup thick cut oats
1/4 cup flour
1/2 cup toasted walnuts
3/8 cup (3/4 stick) butter
Or, ignore these quantities entirely, and use whatever amounts you like. It's hard to go wrong with crumble, so long as it has enough butter and sugar.
Turn oven down to 350, spread crumble topping over fruit and bake for another 20 minutes.