First, prepare the batter. Heat the butter in a microwave or a small pan on the stove just until it is melted. In a medium bowl, stir together the oats, flour, brown sugar, nutmeg and salt. Then, using your nice clean hands, mix in the butter, working everything until it is a crumbly mass with the butter evenly dispersed. If it has little chunks here and there, that's okay.
Prepare an 8x8 baking pan by greasing it with butter. Put 3/4 of the oat batter in the pan and press it down uniformly, again using your hands, so it is uniformly thick.
Preheat oven to 350ºF.
Next, prepare the fruit—do not do this first (See Kitchen Notes)! Chop the fruit, then put it in a small bowl and stir in the sugar, cornstarch and lemon juice. Immediately pour the fruit into the baking pan and lightly spread it around on top of the oat mixture so it's uniform. Do not press it down into place.
Finally, crumble the remaining oat mixture over the top. Put the pan in the heated oven and bake for 35 to 40 minutes until the top is golden and toasty and the fruit juices are a little bubbly. Remove the pan from the oven and let it cool on a rack. Leave it be until it is completely room temperature. That's it! You're done.
The baked bars may be frozen, tightly wrapped.