On a rimmed baking sheet, arrange the chopped potatoes, brussels sprouts, shallot, and salmon. Drizzle everything lightly with olive oil and season with salt and pepper.
Mix the grainy mustard, maple syrup and minced garlic in a small bowl; spread ¾ of the mixture onto the salmon and drizzle the remaining over the potatoes and brussels sprouts.
Roast the ingredients in the oven for 20 to 22 minutes, until salmon reaches desired doneness and potatoes and brussels sprouts have crisped. Serve immediately.