To a medium mason jar, add sliced onion, coriander, sugar, 1 cup vinegar, 1 cup water, and 1/2 teaspoon kosher salt. Store in the refrigerator for at least 24 hours (or up to 1 week) before making the deviled eggs.
In a small bowl, combine beet juice and remaining 1 cup vinegar.
Place eggs in the beet juice mixture and let sit for about 1 ½ minutes, or until they reach the desired pink color.
Remove eggs from the bowl and let them dry on paper towels.
Slice eggs in half lengthwise.
Remove yolks and place into a bowl along with the remaining 1/4 teaspoon salt.
Using a fork, mash yolks into a fine powder.
Stir in the mayonnaise and Dijon mustard.
Mince ¼ cup pickled red onion, then add to the yolk mixture.
Stir until well combined, tasting and adjusting seasonings as desired.
Using a spoon or piping bag, fill the dyed egg white halves with the yolk mixture.
Stick 1 piece of pickled onion onto each deviled egg.
Garnish eggs with mint leaves and coriander seeds from the pickling liquid, and serve on a platter.