To make rhubarb puree:
To make the rhubarb puree, first prepare your rhubarb. If you are using it fresh from the garden, cut off the leaves and the bottoms of the stalks, and wash the trimmed stalks. Cut into one-inch pieces. For one batch, measure about 4 cups / 1 liter of cut rhubarb and toss it with 2-3 tablespoons / 30-45 ml of sugar. You can double or triple the amounts if you want to have extra puree available in the fridge.
Preheat the oven to 375 degrees F / 191 degrees C. Place the prepared rhubarb on a parchment-lined cookie sheet, and roast for 25-30 minutes, or until the rhubarb has released some juice, but hasn't started to caramelize. Remove from the oven, and cool to room temperature.
Softening the rhubarb by roasting rather than simmering with a bit of water on the stove top will give you a more concentrated flavor.
When it has cooled, puree the rhubarb in a food processor or in a food mill. (If you don't have either of these, you can mash the rhubarb thoroughly with a spoon or whisk and put it through a strainer to make sure it is smooth.) Use your puree right away in this recipe, or store it in the fridge for three to four days until you need it.
You can also prepare the cake in a 9x13-inch / 23x33 cm baking pan instead of a bundt pan. Let it bake for about 25 minutes or until golden brown and a toothpick tests clean.