In a mixing bowl, combine the flour, oats, soda, salt, sugars and walnuts, if using. Add to the melted butter and vanilla to the oat mixture and stir to incorporate. When the crumb mixture is evenly moist, press 2/3 of it into the bottom of your prepared baking dish. Set aside the remaining 1/3 of the crumb mixture for the top layer.
1 1/2 cups (212g) all-purpose flour, 1 1/2 cups old-fashioned rolled oats, 1/2 teaspoon baking soda, 1/4 teaspoon salt, 1/2 cup packed light brown sugar, 1/2 cup (100g) granulated sugar, 3/4 cup chopped walnuts, 3/4 cup salted butter, 2 teaspoons pure vanilla extract