Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
Toss the broccoli florets with a drizzle of olive oil, salt, and pepper. Spread them on the baking sheet.
Roast the broccoli in the preheated oven for about 15-18 minutes, until the edges start to crisp.
Meanwhile, steam or boil the cauliflower until very tender, about 10 minutes.
Drain the cauliflower and mash it with the nonfat Greek yogurt, remaining olive oil (if desired for extra creaminess, though already used mostly for the broccoli), a squeeze of lemon juice, salt, and pepper until smooth.
Season the salmon fillet with salt, pepper, and a light sprinkle of garlic powder if desired.
Heat a non-stick skillet over medium-high heat. Sear the salmon, skin-side down if applicable, for about 3-4 minutes per side, until a golden crust forms and the fish flakes easily.
Plate the salmon alongside the roasted broccoli and a generous serving of the cauliflower mash. Finish with an extra drizzle of lemon juice and a light sprinkle of black pepper.