Yum, Yum, Yum. Get over it... Prunes are delicious! I found this recipe and love it. The pie crust recipe wasn't included, and I'm going to second that. I'm sure someone else has posted this most important of staples for any chef.
Make the crust and pre-bake it using any unsweetened buttery crust recipe to produce a crisp crust.
Boil enough water to cover the prunes in a bowl. Cover them and let sit for twenty minutes. Chop prunes coarsely.
Preheat the oven to 350°F.
Sauté the onions over medium heat in a large sauce pan in the oil. Salt lightly. A lot of water will come out of the onions. Either cook it away or drain after ten minutes and add the balsamic. Toss to coat and continue cooking the onions. Cook until liquid is absorbed and onions are meltingly soft.
Mix the mustard powder and prunes into the onions well and heat through. If you are so inclined, you could add a few minced anchovy filets at this point.
Dump the whole mess in the pie crust and crumble the blue cheese all over the top of the tart. Bake for 25 minutes in a 350°F oven or until the cheese takes on a bit of color and the crust is crispy.
Nice and sweet with no added sugars. Everything sweet you taste is from the balsamic and the onions. The blue cheese (and optional anchovy) cuts through the sweetness.