Prepare the dough by combining flour, water and salt and mixing in a stand mixer on low speed with a dough hook attachment for 5 minutes. This type of dough is very stiff so you must use a powerful, commercial grade mixer. If mixing by hand, combine all ingredients in a large bowl and let the flour hydrate for 5 minutes or longer, then knead for 5 minutes. Wrap in plastic and let sit at room temperature for at least half an hour.
Prepare the meat filling by combining the meat, the onions and the seasonings.
Transfer the dough to a lightly floured surface. Roll it out to a rectangle about 1/16 inch thick. Pour 3 tablespoons of vegetable or olive oil and spread out evenly across the entire surface. Roll the dough into a tight log, making sure there are no air pockets. Slice the log into about 16 1-1/4 inch cylinders. Flatten each cylinder with the palm of you hand, then roll out to a 5 inch circle.
Put about 3 - 3-1/2 ounces of meat filling in the center of each circle. Seal as shown in the picture instructions above and place on a large baking sheet seam side down.
Whisk egg yolks and milk until perfectly mixed. Using a pastry brush paint the sambusas with a light, even coat of egg wash. Sprinkle roasted sesame seeds on top of each sambusa.
Bake on the top rack of the oven preheated to 460F for 30 minutes, turning the baking sheet around once after 15 minutes. For softer crust, bake at 425F for 30 minutes.
Let cool for 5 minutes and serve.
Refrigerated leftover sambusa can be re-heated for 10-15 minutes in the oven at 425F.