Make the multi-seed oat mixture: in a small bowl combine the rolled oats and all the seeds. If you can't find each variety of seeds, it's ok. I think the two most important texturally are the sunflower seeds and the rolled oats. The poppy seeds come into play in the appearance department but if you just have those three seeds/oats you will be ok.

Once they are all combined, place them in a sealed container in a cool spot in your kitchen and they will last for a few months at least (probably a lot more).

Dough: in a large bowl or the bowl of your mixer, weigh out and add your all-purpose flour, sugar, salt, multi-seed/oat mixture, and instant yeast and stir to combine all dry ingredients making sure there are no clumps. 

Add the butter and room-temperature water to the dry ingredients. 

If using a stand mixer, knead for 8 minutes on medium speed. If you are kneading by hand, you will need about 15 minutes of kneading after all the ingredients are incorporated together. 

The dough should be smooth and a little bit sticky. 

Place your dough in a lightly oiled bowl and cover. Let dough rise in a warm place for 1 to 1.5 hours. The dough should be doubled in size. 

Remove dough from bowl to a lightly oiled counter and divide into 6 pieces. I like to weigh my buns at this point to keep them all fairly consistently sized, and I usually get 6 buns that are all between 85 and 90 grams each. 

Roll each of your six pieces of dough on the counter until you have 6 balls. 

Add each ball to a parchment-lined sheet pan. 

Press down the top of each ball with a lightly oiled hand or the lightly oiled bottom of a mixing cup or small pan. If you want very tall and round buns, don't flatten them at all. If you want medium-height but still rounded buns press down only a little. If you want flat buns as you see in fast food restaurants, you should flatten each one fairly flat. 

Cover your buns with lightly greased plastic wrap or a second sheet pan flipped over on top of the pan you're using. 

Let the shaped buns rise for another hour or an hour and a half until they are nearly doubled in size.

Near the end of your final proofing time, preheat your oven to 375 degrees F (190 C). 

Crack and whisk 1 whole egg with a tablespoon of water. Paint this on top of each bun carefully. Then sprinkle some rolled oats on the top. The egg wash will make the oats stick to the bun, but it will also give the exterior of the bun a browner and shinier appearance.

Bake your buns for 15 to 20 minutes rotating the sheet pan halfway through the baking time. I typically bake for 7 minutes and then rotate and come back in another 7 minutes to see what my level of brownness looks like. At that point your buns should be done all the way through, you're just trying to get them to the color you prefer. 

Remove the buns from the oven and allow them to cool before cutting or storing.