Preheat oven to 350ºF (180ºC). Beat butter with both kinds of sugar until light and fluffy. Beat in egg and vanilla. Set aside.
In separate bowl, whisk oats with all-purpose flour, whole-wheat flour, baking soda, and salt. Stir flour mixture into butter mixture until combined. Mix in pumpkin seeds, sunflower seeds and flax seeds. Cover and refrigerate dough 30 min.
Drop dough by rounded tablespoons, 2 in. (5 cm) apart, onto parchment paper-lined baking sheets. Bake, in batches, 12 min. or until golden brown around edges.
Cool on baking sheet 2 min. before transferring cookies to wire rack to cool completely. Store in airtight container for up 2 days or freeze up to 2 weeks.
Add 1/2 cup (125 mL) chopped dried fruit, such as dried cranberries, raisins or sour cherries to dough.