Preheat the oven to 200°c/180fan/gas 4/350f.
Scrub the potatoes and slice in half - quarter any larger ones.
Add the potatoes to a large pot, cover with boiling water and a pinch of salt. Bring to the boil and simmer for 10 minutes before draining.
While the potatoes are cooking, preheat a large roasting tin in the oven.
Peel the red onions and cut into wedges. Trim the ends of the broccoli stems.
Make the Wholegrain Mustard dressing by combining the mustard, lemon juice, oil and garlic in a small bowl. Add a pinch of salt and pepper and mix.
After draining the potatoes, add them to the preheated roasting tin along with the broccoli and onion wedges. Pour over the mustard dressing and toss everything together.
Lightly season the salmon fillets on both sides with a little salt and pepper and a drizzle of oil. Lay the salmon on top of the vegetables in the roasting dish, skin side up.
Put the roasting tin back in the oven and cook for 12-15 minutes, until the salmon is cooked through and opaque. To check if the salmon is cooked, lightly press on the top of the fillet with a fork - it should flake apart easily along the white lines on the fillet.
Garnish with some chopped fresh herbs and serve - I like a mix of parsley and dill.