Open-Face Eggplant Sandwiches with Ricotta and Pickled Onions

Brush eggplant rounds generously with oil on one side. Heat a large skillet, preferably nonstick, over medium-high. Working in batches if needed, cook eggplant in skillet, oil side down, in a single layer, until well browned, about 4 minutes. Brush each round with oil, turn over, and cook until other side is browned and eggplant is tender, about 3 minutes. Transfer to a baking dish or wide shallow bowl.