For the duck: Juice three of the oranges and cut the remaining two into supremes by cutting away the peel, then cutting the segments free from the white pith. Set aside.
Score the duck skin and place skin-side down in a saute pan over medium-low heat to render the fat until the skin is browned and crispy, about 6 minutes. Turn the breast and cook to medium-well, 5 to 6 minutes. Pour the rendered duck fat into two large saute pans, leaving a little behind in the original saute pan.
Add fennel to the original pan, reserving the fronds, and cook over medium-high heat until soft and translucent. Add chicken stock, white wine, thyme, blood orange juice, red pepper flakes and a pinch of salt and cook, stirring occasionally, until the liquid has reduced by half.
For the red potatoes: Bring a large stock pot of salted water and garlic to a boil. Add the potatoes and cook until fork tender.
Add the onion to one of the saute pans and cook over medium-high heat, stirring occasionally, until caramelized, about 15 minutes. Mix in potatoes and rosemary and cook until potatoes have a nice brown exterior, 3 to 4 minutes.
For the broccoli rabe: Add garlic to the third saute pan and cook over medium heat until the oil is aromatic. Add the broccoli rabe and toss to coat with the garlic oil. Season with salt.
Serve duck breast with potatoes and broccoli rabe. Garnish dish with pan sauce, fennel fronds and supremed blood oranges.