Preheat oven to 400°.
Use a rimmed baking sheet, you can line with foil for easy clean up.
Place salmon on cutting board and drizzle lemon juice over top of salmon and a pinch of kosher salt and black pepper.
In a bowl toss fennel, onion, mushrooms, olive oil and salt and pepper.
Spread the vegetables evenly and roast for 15 minutes. Toss bok choy in empty bowl and coat with any remaining oil that is in the bowl.
Remove pan and toss bok choy over top top of veggies, cook an additional 6-10 minutes.
Remove pan from oven and push vegetables into a pile to make a bed for the salmon.
Place the fillets on top of vegetables, spaced evenly.
Return pan to oven and roast for 10-15 minutes