Preheat oven to 350°F (177°C). Generously spray a 12-count muffin pan with nonstick spray. If using muffin liners, spray them with nonstick spray. The oatmeal cups stick to the liners a bit regardless, so I recommend skipping them.

In a large bowl, whisk all of the ingredients together, except for the apples (and nuts and coarse sugar). Using a spoon or silicone spatula, fold in the apples and nuts (if using). Batter will be a little liquidy. Spoon batter evenly into muffin cups, making sure both the oats/apples and liquid are in each. Fill all the way to the top. See photo above. If desired, sprinkle the tops with coarse sugar.

Bake for 28-30 minutes or until the edges are lightly browned and top appears set. If the tops are browning very quickly, tent a piece of aluminum foil over the muffin pan.

Cool for 5-10 minutes before serving.

Cover leftover oatmeal cups tightly and refrigerate for up to 1 week.