If you're making this the same time that you will be baking it, preheat your oven to 350F.
Start by melting the butter over low to medium heat on the stove top. Then, allow to cool slightly while mixing the rest of your ingredients. Mix the oats, sugar, cinnamon, eggs, kefir or milk, salt, vanilla, and baking powder. Then, mix in the melted butter. Last, add your mix-ins. I added raisins and chopped nuts. Around the holidays, I make cranberry apple baked oatmeal. Here is that recipe for a nice holiday breakfast.
If making this the night before, add all of your mixture to a 9x13 pan, cover, and place in the refrigerator.
When ready to bake the oatmeal, preheat your oven to 350F, add to a 9x13 pan, and bake for 30 to 40 minutes. If your oatmeal mixture is cold from the fridge, it will take closer to 40 minutes to bake. Bake until the top is lightly golden brown. When serving, I like to top the baked oatmeal with a dollop of yogurt. Enjoy your breakfast baked oatmeal!