Ensure the skin of the duck breast is dry, and then cut a cross-hatch pattern (1cm) in the skin, making sure not to cut the flesh.
Place the coarse sea salt, fennel seeds and black pepper in a spice grinder or pestle and mortar and grind.Rub this fennel mix into both sides of the duck breasts.
Place the duck breasts skin side down into a cold 28cm or 11" frying pan or skillet (do not use non-stick). Turn on the heat to a low-medium temperature and allow it to sit for 8-12 minutes.
Peel and cut the red onion in half, and then slice as finely as you can into half-moon shapes.
Cut the top and bottom from the orange and then peel it with a sharp knife, removing the skin just below the pith.Cut each segment from the orange by sliding a sharp knife alongside each membrane.
Squeeze the orange juice from the leftover orange "carcass" into a bowl.
By now, the duck breast should have started to give off some fat; if there is a lot, pour it off into a bowl, and we will be using it later.
Add three times as much oil as you have orange juice from the leftover orange and whisk, season with salt and pepper to taste.
After the duck is nicely golden on the skin side, flip it over and cook it until your desired level of "done". I remove the duck at 53-54°C or 127°F and rest for 5-10 minutes tented in foil.*This results in pink duck. If you want temperatures for other levels of "doneness", see my FAQ section*
Cut the greens from the radish and then cut them in half and place them into a small roasting pan with the asparagus tips and pour over the duck fat. Place the pan in a hot oven at 220°C or 450°F and cook for the time it takes for the duck to rest.
I like to sear the orange segments to add a slightly bitter edge. I use a blow torch, but sliding them under a hot grill or broiler works just as well.
Once the duck has rested, thinly slice it.
Dress the rocket and red onion with the orange juice and oil mix and add to a plate.
Scatter over the roasted radishes, asparagus and seared orange segments.