Store pomegranate arils covered and refrigerated for up to 3 days¸ or freeze for up to 6 months.
Line a large baking sheet with parchment. Spread oats on baking sheet and place in oven. Preheat oven to 375°F. (Do not remove baking sheet while oven preheats.) Leave baking sheet in oven until oats are lightly toasted, about 6 minutes. Transfer to a large bowl and let cool for 5 minutes. (Do not turn off oven.)
Add all-purpose flour, spelt flour, baking powder and salt to bowl with oats; whisk until combined. Cut butter into flour mixture with a pastry blender or 2 knives until mixture resembles coarse meal. Add pomegranate seeds; toss until combined and well coated with flour mixture. Add milk, honey and vanilla, stirring until just combined. (Dough will be sticky and slightly wet.)
Turn dough out onto a lightly floured surface; knead gently 4 or 5 times with floured hands. Pat dough into a 10-inch circle on lined baking sheet. Cut dough into 10 wedges, cutting into but not through dough.
Whisk together egg and water in a small bowl. Lightly brush egg wash over dough; discard remaining egg wash. Sprinkle dough evenly with sugar. Bake scones until golden, 25 to 28 minutes.