Preheat oven to 400 degrees.
Add ground beef to a bowl with 1/2 cup milk. This makes the beef so tender! If you have time, leave it in the fridge the night before to let the beef soak in the milk. Add the parsley, garlic powder, red pepper flakes to the meat. Take the bowl out of the fridge 45 mins before cooking to allow to come to room temperature. This ensures even cooking to the meat.
Slice sweet potatoes (leave skin on) into 1/4-1/2 inch rounds. You'll want 20 slices for the sweet potato buns. Place on a baking sheet covered with parchment paper. Bake for 30 mins (or until tender) flipping halfway.
Slice an onion lengthwise. Heat a cast iron pan to medium-low on the stove top and add 1 tbsp of olive oil. Add the onion to the pan and cook until caramelized. Turn off the heat and add 1-2 tsp balsamic glaze. Allow onions to cool for a few minutes.
Add the caramelized onions to the ground beef mixture and 1/4 cup of crumbled feta cheese. Divide mixture into 10 slider patties on a plate with parchment paper (easy clean-up).
You can use the grill or the cast iron (then finish off in the oven) and right before cooking generously sprinkle sea salt and pepper to both sides of the burger patties.
Use the same cast iron skillet on medium heat and added the burger sliders in two batches (I actually have a HUGE cast iron skillet I can fit all of them on!) Cook for 5-7 mins flipping halfway. You just want a nice sear to the burgers. Bake the burgers in the oven for 15 mins in the cast iron pan or transfer to a large baking sheet covered with parchment paper.
Assemble the sweet potato buns, add the burger sliders and top with avocado and more feta or goat cheese! Add a slice of tomato or get fancy with some bacon!