Heat a griddle over high heat. Sear the lemon slices on each side until they become slightly caramelized. Set aside. When ready to serve, chop the beef filet as finely as possible by hand using a sharp knife. Form the chopped meat into loose balls, about 1 ounce each.
Place the seared lemons on a plate and top with the filet tartare balls. Serve immediately with lemon zest and sea salt as accompaniments.