Preheat the oven to 450 degrees F.
Toss the cauliflower and potatoes with 1 tablespoon grapeseed oil and some salt and pepper. Transfer to a baking sheet and roast until tender and browned, 5 minutes. Set aside.
Pat the salmon fillet dry with a paper towel to remove moisture and prevent sticking to the pan. Sprinkle with salt and pepper.
Heat 1 tablespoon grapeseed oil in a saute pan over medium-high heat. Add the fillet and sear for 2 minutes. Flip, add 1 tablespoon of the butter and then remove from the heat. Baste the salmon with the melted butter. Set aside.
Heat 1 tablespoon grapeseed oil in a second pan over medium-high heat. Add the roasted cauliflower and potatoes; saute for 2 minutes. Deglaze with the white wine, then add the saffron, capers, parsley, chicken stock, lemon juice and the remaining 1 tablespoon butter. Reduce down for 1 minute to make the sauce.
Lay the cauliflower and potatoes on the plate, top with the salmon and the sauce.