I made these:
Okay, so you're wondering, okay Ani... I get it, but whats inside???
I took about 2 pounds of beef, added salt and fresh ground black pepper, cumin, a little Worcestershire, and I grated 3 cloves of fresh garlic. I also added grated parmigiana cheese, probably less than 1/4 cup.
I took two small onions and chopped them and caramelized them (I should have chopped them finer and maybe a little less than 2 small onions - next time).
I chopped a small bunch of fresh basil and added that to the ground meat as well. Once the onion was caramelized and cooled (you don't want to cook the meat before you grill is) I added it all together and kneaded the meat until well blended.
By this time the grill was nice and hot, I weighted out the burgers to be 6 ounces each and came up with 6 burgers. I formed two of them leaving 4; for four burgers I spooned into a bowl 3 spoons of crumbled feta and a small helping of shredded mozzarella.
I flattened out the burgers one at a time and spooned a large spoon of the cheese in each, sealed it and put it aside. Next time I will have to remember to flatten out the burgers more.
I grilled the burgers about 4-5 minutes a side and voila!
These were honestly the best burgers ever. Not just my opinion. Four of us were mmmming and aaahing!! :-)
Let me know how it comes out if you try!