Peel 2 oranges all around, also removing the white skin. Collect the juice in a bowl. Cut the oranges into ½ cm thick slices, remove any stones that may be present. Spread the orange slices on a large plate.
Clean the fennel bulb, remove the hard outer parts. Set aside the fennel greens for garnish. Cut the tender fennel heart into small cubes. Cut 2 onions into very fine rings.
Whisk the orange juice with 1 tablespoon white wine vinegar and 4 tablespoon olive oil. Finely chop 1 teaspoon rosemary, mix in. Season to taste with salt and pepper.
Turn the diced fennel thoroughly in the sauce, remove and distribute on the orange slices.
Scatter onion rings, drizzle the remaining sauce over the salad. Pepper roughly from the mill, garnish with fennel greens.