Preheat the oven to 350 degrees F. Generously butter a 3-quart baking dish.
Combine the stock, rice blend, quinoa and 1 1/2 teaspoons of salt in a large pot. Bring to a boil, stir once, and cover. Reduce the heat to low and cook until the grains are tender and most of the liquid has been absorbed, about 18 minutes. Remove from the heat.
Meanwhile, melt the butter in a large skillet over medium heat. Add onions and cook until very soft, 10 minutes, and then stir in the parsnips and garlic and cook until nicely browned, 7 minutes. Add the mushrooms and thyme and cook, stirring, until the mushrooms are tender, 5 minutes.
Combine the grains, vegetable mixture, apricots, if using, parsley, 1 teaspoon salt and 1/4 teaspoon pepper in a large bowl. Pour into the prepared baking dish and bake until the top is nicely toasted, 25 to 30 minutes. Serve hot.