Mix the wet ingredients: Preheat the oven to 375°F (190°C) and lightly grease a 12-cup muffin pan.In a large bowl, whisk 1 cup Greek yogurt, ½ cup unsweetened apple sauce, ¼ cup honey, 1 large egg, and 1 teaspoon vanilla extract until smooth.
Add the oats and apples: Stir in 1½ cups rolled oats, 1½ teaspoons cinnamon, 1 teaspoon baking powder, and ¼ teaspoon salt until all the oats are coated.Fold in 2 medium apples (chopped) and ½ cup raisins, if using.
Fill the muffin pan: Divide the mixture evenly between the 12 muffin cups, filling them to the top. Gently press the mixture down with the back of a spoon.
Bake and cool: Bake at 375°F (190°C) for 23 to 25 minutes, until the tops are lightly golden and the centers feel set.Let the cups cool in the pan for 20 minutes so they firm up. Run a small knife around the edges, then gently lift them out.
Substitutions
Rolled oats → Quick oats or certified gluten-free oats.
Apples → Honeycrisp, Fuji, Gala, Granny Smith, or pears.