Add flax egg and all the dough ingredients in a large bowl, and mix together to create a shaggy dough.
Transfer dough onto a clean surface. Knead the dough for about 15 minutes, or until it becomes soft and smooth, slightly tacky.
Place the dough in a greased bowl and cover it. Keep in a warm place and let the dough rise for 60 minutes, until it's double in size. (The first rise)
Punch the dough to deflate, and divide it into 4 pieces. Shape each piece into a ball. Place the balls on a baking sheet lined with parchment paper, and gently flatten them into a bun shape.
Cover with a towel and let it rise for 45 minutes. (The second rise)
With 15 minutes left of the second rise, preheat the oven to 375℉/190℃.
Lightly brush the top of buns with coconut oil, and sprinkle seeds of your choice. Bake in the oven for about 15 minutes, or until golden brown.
Remove from oven, and allow the buns to cool completely. Slice and assemble a burger with all of the fixings of your choice. Enjoy!