Preheat oven to 200C.
Line a low sided oven tray with baking paper or foil.
In a mixing bowl, combine 2 tablespoons of the olive oil and 1 teaspoon salt with the juice of half the lemon.
Take salmon fillets out of fridge and any wrapping and sit on paper towel.
Prep vegetables: leaving skins on potato and sweet potato, cut each bite size pieces, trim the broccoli stem then cut stem and florets into bite sized pieces.
Toss all vegetables in the bowl then spread out on oven tray and cover with foil. Start baking. Set timer for 25 minutes.
While the vegetables bake, cut the other half of the lemon into four smaller wedges and add them to the bowl with another tablespoon of oil. Put the salmon fillets in the bowl and move around to pick up the oil, juice and salt. Set aside.
When timer goes off, take the tray out of the oven and remove the foil. Push the potatoes towards the edges. Lay the salmon fillets in the centre of the tray with the onion and small lemon wedges. Set time to bake for another 10 minutes, uncovered.