2) Saute the onions and fennel in the olive oil for 10 minutes on medium-high heat, stirring occasionally. Add the thyme leaves, fennel fronds, orange zest, orange juice, 2 tsp of salt and 1 tsp of pepper and saute for 5 more minutes, until the onions and fennel are tender. Taste for salt and pepper.
3) Lay the salmon, skin side down, on a cutting board and sprinkle both sides generously with salt and pepper. Spread the fennel filling over half of the salmon. Pull the other half up and over the filling, enclosing it. Tie the salmon every 5cm with kitchen string to secure the stuffing.
4) Place a baking tray lined with baking paper in the oven for 5 minutes to heat. Carefully transfer the salmon to the hot baking tray and bake for exactly 30 minutes (10 minutes for each 2.5cm of thickness). Do not overbake.
5) Allow to cool slightly, then remove the strings. To serve, cut into thick slices with a very sharp knife. This salmon is delicious hot or at room temperature.
Cook's Note: If you're making this in advance, be sure to cool the filling before stuffing the salmon. Refrigerate until ready to roast.