Preheat the air fryer to 300°F (150°C) or the oven to 325°F (160°C). Line the basket or a baking sheet with parchment paper.
In a large bowl, combine the oats, multigrain blend, nuts or seeds, cinnamon, salt, and protein powder if using.
In a separate bowl, whisk the honey, maple syrup, oil, and vanilla. In a small bowl, whisk the egg white until frothy.
Pour the syrup mixture over the dry ingredients and stir to coat evenly, then gently fold in the frothy egg white.
Spread onto the parchment and press down firmly into a compact, even layer.
Air fryer: cook 12–15 minutes, rotating the parchment once halfway. Oven: bake 20–25 minutes, rotating once, until golden. Watch closely near the end.
Cool completely (30–45 minutes) without stirring — the clusters set as it cools.
Break into clusters and fold in any post-bake add-ins. Store in an airtight container.