Start by preheating your oven. Cut the cauliflower into even pieces and slice the red onion. Toss them with olive oil, salt, and pepper. Spread everything on a baking sheet for even roasting.
Place the tray in the oven and roast until the cauliflower becomes tender and lightly browned. Turn the pieces halfway through cooking. This step builds deep flavor and caramelization.
While the vegetables roast, simmer the black lentils in salted water. Cook until tender but not mushy. Drain and rinse under cool water to stop the cooking process.
In a small bowl, whisk together garlic, mustard, lemon juice, olive oil, salt, and pepper. Mix until the dressing becomes smooth and well combined.
In a large bowl, combine lentils, roasted cauliflower, onions, olives, pistachios, and parsley. Pour the dressing over the top and gently toss everything together.