Preheat the broiler. In a medium mixing bowl, combine the beef, cumin, and red pepper flakes. Season generously with salt and black pepper. Mix to combine the seasonings well, but take care not to overmix (this will make your beef dry, and no one likes dry beef.)
Form beef mixture into 1” meatballs, and place on a parchment lined sheet pan, spaced about 2” apart. Broil on a sheet pan, turning occasionally, until charred all over but still tender, about 10-12 minutes.
While the beef cooks, assemble the watermelon salad. Combine the watermelon with the herbs and onion in a medium bowl. Finish with a glug of olive oil, some flaky salt, and black pepper. Serve with the balls.