Prepare the oven and berries. Heat a conventional oven to 350°F (175°C). Lightly grease an 8-inch square pan at least 2 inches deep. Wash, hull, dry, and cut the strawberries, then weigh the prepared pieces. Let melted butter cool until warm rather than hot so it can be whisked into the eggs without cooking small pieces of them.

Combine the oats. Stir the rolled oats, baking powder, and salt together in the larger bowl. Check that the baking powder is evenly distributed. Keep the oats whole; blending them changes the texture from an oat bake toward a flour-like batter and is not part of this method.

Whisk the liquid mixture. Whisk the eggs in a second bowl, then whisk in milk, maple syrup, cooled melted butter, and vanilla. If a few small butter flecks form because the milk is cold, continue whisking to disperse them. The mixture should be evenly combined before it is poured over the dry oats.

Mix and distribute the fruit. Pour the liquid mixture into the oats and stir thoroughly. Fold in the strawberries, scraping along the bottom to lift any settled oats. Transfer to the prepared pan and spread evenly, distributing the berries across the corners as well as the center. Do not add an extra layer of unmeasured fruit on top.

Bake until the center sets. Bake for about 38–45 minutes. The edges should be lightly golden and the center should be set without a pool of liquid when the pan is gently moved. Check that the egg-containing mixture reaches at least 160°F (71°C) in the center. If the surface colors before the center sets, loosely cover with foil and continue baking.

Rest and portion. Rest the oatmeal for 10 minutes before serving. Cut into nine modest squares, or spoon warm portions from the pan if preferred. For later breakfasts, divide leftovers into shallow containers and refrigerate promptly; do not leave the whole pan on the counter for hours waiting for a very firm slice.