Adjust the oven rack to the lower third position and preheat the oven to 350°F (177°C). Spray a 9×9 inch or 11×7 inch baking pan with nonstick spray. Any similar size or shape pan works, though 8×8 inch would be too small. See recipe note for 9×13 inch pan.
Whisk all of the ingredients together in 1 large bowl. Pour into prepared baking pan. Top with nuts, if desired. (Or stir into the oatmeal.) Bake for 35 minutes or until the center appears almost set, which gives us a soft oatmeal as pictured above. For drier and more solid baked oatmeal, bake until center has set.
Cool for 5 minutes before serving. Spoon or slice and serve with yogurt, if desired. Cover leftovers tightly and refrigerate for up to 1 week.