Preheat the oven to 425°F for roasting the broccoli.
Toss the broccoli with olive oil, salt, and pepper, then spread evenly on a baking sheet.
Roast the broccoli in the oven for 15-20 minutes until tender and slightly crispy on the edges.
Meanwhile, steam the cauliflower until soft (about 10 minutes).
In a small saucepan, melt the butter and sauté the minced garlic for 1 minute until fragrant.
Add the steamed cauliflower to the saucepan and mash together with the garlic butter; season with a pinch of salt and pepper.
Heat a non-stick skillet over medium-high heat. Season the salmon fillet with salt and pepper.
Sear the salmon skin-side down for about 4 minutes, then flip and cook another 3-4 minutes until the fish is just cooked through.
Plate the seared salmon, add a serving of roasted broccoli, and top with a generous scoop of garlic mashed cauliflower.
Finish with a squeeze of lemon juice over the salmon for brightness and serve immediately.