Serves 4–6Place the onions in a heavy-bottomed pan over medium heat. Add the butter and ½ teaspoon of salt. Put a lid on the pan and allow the onions to cook slowly for 20 to 25 minutes, stirring frequently, until soft and translucent. Set aside.Preheat the oven to 450°; if using a pizza stone, place the stone in the oven to warm up for 20 minutes. Roll out the pizza crusts. If using a pizza stone, remove it from the oven and dust with cornmeal. Carefully place each crust on the stone or baking sheet, and spread half the pesto evenly across each crust. Layer half the onions, beets, crumbles of goat cheese and walnuts on each crust and place one pizza in the oven.Bake for 18 to 20 minutes. Remove the first pizza from the oven and place the second pizza in the oven for 18 to 20 minutes. Sprinkle half the thyme leaves over each pizza, drizzle with olive oil and sprinkle with salt and black pepper. Serve warm.