To make the dressing: whisk together the citrus juice, mustard and vinegar. Very slowly drizzle in the olive oil, whisking constantly until all of the oil has been added and emulsified.
To segment the citrus: slice both ends off of the citrus and place one of those side down on the cutting board. Starting at the top of the fruit, cut downwards through the peel and pith, following the contour of the fruit. Rotate and repeat all the way around. Then, using a small paring knife, slice along the membranes towards the center to remove each segment. Squeeze remaining juice from what is left - keep for the dressing if needed.
Dress the greens and either add the citrus to the bowl or for a more elegant presentation, arrange the segments once greens are plated. Finish with breadcrumbs.
Recipe by Eat Some Wear Some at https://eatsomewearsome.com/winter-citrus-salad-w-toasted-zaatar-breadcrumbs/