Prepare the crepes first. Place all of the ingredients into a blender: eggs, milk, butter, salt and flour. Blend for 2 to 3 minutes until the batter is smooth and lump free. Begin with 2 1/4 cups flour, add more if batter is too thin.
Preheat a crepe pan over medium heat and use 1/4 cup batter for each crepe. Pour the batter into the center of the pan, then tilt to spread it evenly to the edges. Cook for 1 to 2 minutes on the first side, until golden, then turn with a rubber spatula and cook on the other side for 30 seconds. Stack the crepes onto a tray.
Prepare the meat filling. Preheat a large saute pan over medium heat and add a drizzle of olive oil. Add the ground beef and season with approximately 1 1/2 teaspoons salt, a bit of ground black pepper and garlic powder. Fry the ground beef for 5 to 7 minutes, until well browned all over, breaking apart any larger pieces. Add the grated onion and fry for 2 to 3 more minutes, until onion is tender and fragrant.
In a large bowl, combine the meat mixture with grated mozzarella cheese and fresh dill. To assemble the blini, spread a few tablespoons of the filling onto the center of each crepe. Fold over the sides first, then fold into a small pocket.
Preheat a clean frying pan over low-medium heat and add about 1/2 tablespoon of butter for each batch. Fry the blini 3 to 4 at a time, 2 to 3 minutes per side, until golden. Serve warm with sour cream on the side.