Whisk together yogurt, milk, water, eggs, sugar, butter, vanilla, and cinnamon in a large bowl. If using puree as a mix-in, add it at this step.
Sprinkle baking powder over oats in a medium bowl and toss or stir to distribute evenly.
Add oats to wet ingredients.
Stir until oats are fully incorporated and the mixture is consistent.
Pour the oat mixture into a lightly oiled 9×13 pan.
Sprinkle mix-ins over the oats, then stir gently to distribute them into the oatmeal.
Bake immediately at 350°F for 1 hour OR cover and place in the refrigerator for up to 24 hours. When ready to bake refrigerated oatmeal, place the cold pan in a cold oven, then preheat the oven to 350°F. Set a timer for 1 hour 10 minutes (including preheating time).
When done, baked oatmeal will be browned and set in the middle.
Serve warm, topped with milk or yogurt. This recipe is also delicious cold, so you don’t need to warm up the leftovers (if there are any) unless you really want to.