Have the fishmonger cut the head and tail off the salmon and butterfly it, removing all the bones. You should have about 7 pounds of salmon.

Preheat the oven to 500 degrees F.

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, salt, and pepper and saute for 5 more minutes, until the onions and fennel are tender. Taste for salt and pepper.

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 2 inches with kitchen string to secure the stuffing.

Place a sheet pan lined with parchment paper in the oven for 5 minutes to heat it. Carefully transfer the salmon to the hot sheet pan and bake it for exactly 30 minutes (10 minutes for each 1-inch of thickness). Do not overbake!

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.

Note: If you're making this in advance, be sure to cool the filling before stuffing the salmon. Refrigerate until ready to roast.