Bring large pot of water to boil, add salt to water just before pasta and boil pasta until a 2 minutes short of done according to directions.
Heat a pan until very hot, add about 2 tbsp olive oil season both sides of salmon with Cajun seasoning and then cook the salmon chunks on first side until browned and cooked half through. It's easy to tell when half-done because the flesh becomes pale as it cooks through. When flesh is lightened half-through, flip and cook another few minutes until just browned on the other side. A really hot pan is needed for this. I prefer my salmon cooked medium and still a little pink in the middle. Remove salmon and if you want cooked through more, place in oven at 350F until cooked through.
In a separate pan heat oil in pan and cook fennel and swiss chard.
In fish pan add garlic, tomato paste and a little water to thin out for sauce. Add vegetables back in and cook through. Add pasta to finish cooking, taste and season with more salt and pepper if necessary.
Serve pasta with salmon on top and a bit of lemon squeezed over at the last minute and some fennel fronds as a garnish.