First, toss the cauliflower florets and sliced red onions with olive oil, salt, and pepper on a baking sheet. Roast them at 400°F (200°C) until lightly browned and tender, which usually takes around 20–25 minutes. Roasting brings out their natural sweetness and adds depth to the salad.
While the vegetables roast, simmer the lentils in a saucepan of water until just tender, about 20 minutes. Drain and rinse with cold water to stop cooking. This keeps them firm and prevents them from getting mushy in the salad.
In a large bowl, mix the roasted cauliflower and onions with the cooked lentils, sliced olives, chopped pistachios, and parsley. This creates a beautiful medley of textures and flavors.
Whisk together olive oil, fresh lemon juice, minced garlic, salt, and pepper. Pour it over the salad and toss everything until well coated. The vinaigrette adds a zesty, bright finish to every bite.