Tip:If entertaining, you can cook and serve the fillet whole.
Position the rack in the center of the oven and heat the oven to 400°F. Rub the salmon flesh with 1 Tbs. oil and then sprinkle with half the thyme, 3/4 tsp. salt, and 1/2 tsp. pepper.
In a large (12-inch), oven-proof skillet (like a cast-iron pan) over medium heat, cook the bacon in 1 Tbs. oil, stirring occasionally, until it crisps and renders most of its fat, about 5 minutes. Transfer to a plate lined with paper towels. Discard the bacon fat and wipe the pan with a paper towel.
Add the remaining Tbs. oil and the leek to the pan, sprinkle with 1/2 tsp. salt, the remaining thyme, and the vinegar, and cook over low heat, stirring occasionally, until the leek softens completely and turns light brown, about 10 minutes.
Spread the leeks evenly around the pan and set the salmon skin side down on top. Transfer to the oven and roast uncovered until the salmon browns and becomes mostly firm to the touch (it should be just a little pink inside when you flake open a thicker piece of the fish), about 20 minutes.
Serve the salmon with leeks and crisp bacon on top.