Preheat oven to 425°F (220°C). In a large bowl, mix 1 cup (250 mL) flour with the oats, poppy seeds, sugar, baking powder and 1/4 tsp (1 mL) salt.
Reserve 1 tsp (5 mL) butter; add the rest to the flour mixture and combine either by cutting together with two knives in a criss-cross motion or using fingertips to quickly rub together until the mixture resembles coarse meal. Use a fork to stir in buttermilk, a small splash at a time, until a slightly sticky dough forms.
Dust countertop with remaining flour. Turn dough onto countertop and pat into a 7-in. (18 cm) circle about 1/2 in. (1 cm) thick. Cut into 8 wedges. Place evenly spaced on an ungreased baking sheet. Bake 15 to 20 min. or until just golden.
Meanwhile, melt reserved butter in a skillet over medium heat. Add onions and sauté until sizzling and just beginning to brown. Add egg whites and scramble. Season with remaining salt.
To assemble, split 4 baked oatcakes crosswise. Fill each with scrambled egg whites, sour cream and a sprinkling of dill. Remaining oatcakes should be tightly wrapped in plastic, kept in the refrigerator for up to a day and reheated in the microwave.