Preheat the oven to 325ºF (170ºC).
Spray or butter a 9-inch square baking pan (23cm) and line it with parchment paper for easier removal of the bars. See Notes below.
Stir to combine 1 ½ cups all-purpose flour, 1 ½ cups traditional oats, ¾ cup brown sugar, ¾ teaspoon baking soda and pinch of salt in a large bowl. Add ¾ cup unsalted butter, melted, and mix with a spatula or wooden spoon until it’s all wet and no dry parts remain.
Reserve 1 ½ cups of this mixture (loosely measured not packed), and press the remaining crumb mixture onto the bottom of the prepared pan.
Bake for 10 minutes, until it starts to dry and color a bit. You don't have to bake it fully.
Take it out of the oven and immediately sprinkle 1 cup semisweet chocolate chips or chunks over the hot base.
Sprinkle ¾ cup walnuts on top of the chocolate.
Drop 1 cup dulce de leche or thick caramel sauce by spoonfuls on top of the walnuts. It will not cover the whole surface. That’s fine.
Sprinkle the reserved oat mixture and very lightly press it down. You don't want to compact it, just make it slightly adhere to the rest of the ingredients.
Bake for an additional 15-20 minutes, until it starts to color. Don't overbake it.
When you take it out of the oven, run a smooth knife along the sides to separate any dulce de leche that might have stuck and make it hard to unmold later.
Let cool completely on a wire rack. Lift the paper to take out the whole block from the pan and cut it into squares or rectangles.
Eat at room temperature.
Store leftovers covered in plastic wrap or in an airtight container at room temperature for a few days, or refrigerated. They can be frozen for a month, well wrapped.