Marc Matsumoto from Public Broadcasting Service and Laura Calder from Cooking Channel TV both offer recipes for baking onions that involve baking the onions and then covering them in a dressing. The baking time and the composition of the dressing varies by recipe.
Laura Calder's recipe makes four onions. To begin, cut a sliver off the bottom of each onion before placing them on small piles of salt in a pan. Bake them in the oven at 425 degrees Fahrenheit for 15 minutes. Next, lower the heat to 325 F and bake for 90 minutes, or until the insides are soft like applesauce. After baking, cut a slit into each onion, and spoon a dressing into the gap until it bubbles out onto the plate.
The dressing consists of 1 egg yolk, whisked together with 1/2 clove of minced garlic and 1 teaspoon of tarragon vinegar. Season with salt and pepper, and then beat with 1/2 cup of grapeseed oil to thicken the mixture. Add a handful of chopped thyme and refrigerate. The onions should be served while hot, immediately after the dressing has been added.
Marc Matsumoto's recipe makes two onions and uses only balsamic vinegar, olive oil and sea salt. Matsumoto suggests baking the onions at 450 F for 40 to 50 minutes, or until the juices running out of the onion begin to char. After removing the onions from the oven, allow them to cool before handling to avoid burns from the juice, and then slice them in half. Finally, drizzle them with the oil, vinegar and salt, and serve.