1
: Combine oats, flour, baking soda and salt; set aside.
2
: Cream together butter and sugar; add reserved oat mixture, mixing until crumbly. Reserve 1 cup (250 mL) crumb mixture, press remainder on bottom of parchment paper-lined 13 x 9" (3 L) baking dish. Bake in preheated 350°F (180°C) oven for 10-12 minutes.
3
: Whisk together remaining ingredients except walnuts; pour over prepared crust. Stir walnuts into reserved crumb mixture; sprinkle over top. Bake 25-30 minutes. Cool. Cut into squares and garnish as desired.