Slice onion in half lengthwise, then in very thin slices crosswise to form half circles.
Cut fennel bulb in half lengthwise and remove core; thinly slice crosswise to form half circles.
Place onion and fennel in a non-reactive bowl and sprinkle with 2 tsp salt.
Toss and let stand for 4 hours. Rinse twice and drain thoroughly.
Combine vinegar, water, and sugar in a medium size dutch oven. Bring to a boil over high heat.
Add vegetables and return just to a boil, stirring constantly. Remove from heat.
Sterilize jars then add peppercorns to the bottom. Remove vegetables from liquid with a pair of tongs and pack into hot jars using your plastic spatula.
Pour liquid over vegetables leaving a 1/2 inch headspace. Clean rim, apply lids and rings.
Process 10 minutes for half-pint jars and 15 minutes for pint jars.
All good Relish's take about two to four weeks to meld flavors. I will keep you posted. Below are my three bibles: