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1
Heat the oven to 400u00b0F. Wrap the beets loosely in aluminum foil and roast until easily pierced with a fork, 50 to 60 minutes. Set aside to cool.
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2
Meanwhile, bring a 2-quart pot of water to a boil. Salt the water generously and add the rice. Reduce the heat to a simmer and cook the rice until it's a little beyond al dente. You want it a little over-cooked, but still firm (not completely mushy). This should take about 35 to 40 minutes. Drain the rice and set it aside to cool.
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3
Heat a teaspoon of olive oil in a skillet over medium-high heat. Add the onions and a pinch of salt. Stir the onions every minute or two, and cook until they are golden and getting charred around the edges, 10 to 12 minutes. A few wisps of smoke as you are cooking is ok, but if it seems that the onions are burning, lower the heat. A dark, sticky crust should develop on the bottom of the pan.
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4
Add the garlic and cook until it is fragrant, about 30 seconds. Pour in the cider vinegar and scrape up the dark sticky crust. Continue to simmer until the cider has evaporated and the pan is nearly dry again. Remove from heat and set aside to cool.
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5
Process the oats in a food processor until they have reduced to a fine flour. Transfer to a small bowl and set aside.
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6
Drain and rinse one of the cans of beans and transfer the beans to the food processor. Scatter the prunes on top. Pulse in 1-second bursts just until the beans are roughly chopped - not so long that they become mush - 8 to 10 pulses. Transfer this mixture to a large mixing bowl. Drain and rinse the second can of beans and add these whole beans to the mixing bowl as well.
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7
Use the edge of a spoon or a paper towel to scrape the skins off the cooled roasted beets; the skins should slip off easily. Grate the peeled beets on the largest holes of a box grater. Transfer the beet gratings to a strainer set over the sink. Press and squeeze the beet gratin
