POSITION rack in centre of oven, then preheat to 325F.
Line a large baking sheet with parchment.
TOSS oats with pecans, pepitas, coconut, flaxseed, butter and salt in a medium bowl until well coated.
Spread on prepared sheet.
BAKE until pecan mixture smells fragrant, 10 to 12 min. (It’s okay if mixture isn’t toasted and golden-brown. It will continue to toast as it bakes in the cookies.)
Remove from oven and stir in blueberries. Let cool.
BEAT egg white in a medium bowl using an electric mixer until foamy, about 1 min.
Beat in sugar until soft peaks form, about 2 min.
Fold in pecan mixture until evenly coated.
PACK 1/4-cup portions onto same baking sheet.
With damp hands, lightly shape and press portions into 1-in. thick rounds.
BAKE, rotating sheet halfway through, until edges are golden and crisp, 20 to 25 min.
Let cool 10 min on sheet, then transfer to a wire rack to cool completely.
Cookies will keep well in a sealed container on the counter for up to 5 days.