Thirty minutes before baking the pizza, preheat the oven to 500 degrees. Heat the olive oil over medium heat in a large, heavy skillet. Add the onions. Cook, stirring often, until tender and just beginning to color, about 10 minutes. Add the thyme, garlic and a generous pinch of salt. Turn the heat to low, cover and cook another 10 to 20 minutes, stirring often, until the onions are golden brown and very sweet and soft. Remove from the heat.
While the onions are cooking, stem and wash the chard leaves, and bring a medium pot of water to a boil. Fill a medium bowl with ice water. When the water comes to a boil, salt generously and add the chard. Blanch for one to two minutes, just until the leaves are tender, and transfer to the ice water. Drain and squeeze out excess water. Alternatively, steam the chard for two to three minutes until wilted, and rinse with cold water. Chop the chard medium-fine.
Roll out the dough, oil a 14-inch pizza pan and dust with cornmeal or semolina. Place the dough on the pan.
In a medium bowl, combine the ricotta, egg yolk, Parmesan and chard. Spread over the pizza dough in an even layer, leaving a 1-inch border around the rim. Spread the onions over the ricotta mixture.
Place in the hot oven, and bake 10 to 15 minutes until the crust and bits of the onion are nicely browned. Remove from the heat, and serve hot or warm.