Preheat the oven to 425 degrees F. Put on a large pot of water to boil for the kale. Put the beets in a medium pot; cover with water, and bring to a boil. Boil the beets until tender when pierced with a paring knife, about 10 minutes, depending on size. Drain, and run cold water over each beet, rubbing off the skin. Slice the beets into 1/4” slices. Drop the kale leaves into boiling water and stir, cook for about 2 minutes, until softened. Drain, rinse with cold water, then wring out until very dry. Chop the drained kale, put in a medium bowl and toss with olive oil and salt.
Assemble the pizzas. Place the pitas on two sheet pans and divide the kale between the four breads, patting it down. Cover with beet slices, distribute chunks of ricotta over the beets, and sprinkle with walnuts.
Bake for 15 minutes, until pitas are crisp and there are golden spots on the cheese.