Heat a skillet over medium to medium-high heat, add olive oil, 2 turns of the pan, then add onions and garlic and cook 5 minutes. Remove to bowl and cool.
Combine the cooled onions and garlic with parsley, zest and juice of a lemon, and chili flakes. Combine the onion-gremolata mixture, beef, and salt and pepper. Form 4 patties, thinner at the center and thicker at the edges.
Heat a cast-iron skillet over medium-high heat.