Heat the oven and prep the pan: Preheat your oven to 350°F (175°C). Lightly grease a 9x9-inch baking dish or similar.

Mix the wet ingredients: In a large bowl, whisk together the eggs, milk, sourdough starter, maple syrup, melted butter, and vanilla until smooth.

Add the dry ingredients: Stir in the oats, cinnamon, salt, and baking powder. Fold gently until the oats are evenly coated.

Stir in add-ins: Fold in fruit, nuts, or chocolate chips if using. If adding very juicy fruit, reduce the milk by a couple of tablespoons to keep the texture set.

Rest the batter (optional but helpful): Let the mixture sit for 10–15 minutes. This gives the oats time to hydrate and leads to a creamier bake.

Transfer and bake: Pour the mixture into the prepared dish and smooth the top. Bake for 35–45 minutes, until the center is set and the edges are lightly golden.

Cool slightly: Let it cool for 10 minutes. It will firm up as it rests, making it easier to slice.

Serve: Scoop warm with a spoon for a soft, pudding-like texture or cut into squares for a more structured slice. Add yogurt, a drizzle of maple syrup, or a dollop of nut butter on top.