Heat one tablespoon of the oil in a nonstick frying pan. Season the salmon and fry for 3 minutes skin-side up. Transfer to a plate.
Fry the bacon for 4-5 minutes. Add the red onion, fry for 2 minutes, then add the mushrooms, cook for 5 minutes more and stir in the lemon zest.
Put the salmon on top of the onion mixture in the pan (or use an ovenproof dish) and broil on high for 2-3 minutes. Mix the vinegar, the rest of the oil and the rosemary with a pinch of sugar and season.
Spoon over the salmon. Good with celeriac mash or crispy roast potatoes.