Dice the whole cauliflower crown into medium-large size florets. In a large mixing bowl, combine with olive oil, salt, and turmeric powder. Gently toss to coat well.
Prepare a large unlined sheet pan. Once the oven reaches the temperature, place it in the oven for 5 minutes in the middle rack.
Carefully remove the sheet from the oven with an oven glove, pour and spread the cauliflower florets over the sheet. Use a tong to separate each florets and place them cut-side-down so they aren’t overlap with each other. Roast for 25 minutes total. Flip after 15 minutes.
In the meantime, make the lemon tahini sauce. Stir-well until creamy smooth. Add more water or lemon juice for thinner texture.
Lightly dry toast pine nuts in a skillet over a stovetop for about 1-minute. Toss often. Roughly chop baby arugula. Set them aside ready to use.
To serve, spread the lemon-tahini sauce over a medium-large size serving plate. Use a back of spoon to spread the sauce thin and evenly. Add roasted cauliflower on top of the sauce. Garnish with pine nuts and arugula and drizzle with more olive oil and lemon juice. Serve warm or in room temperature.
You can make the sauce 1 day ahead. Thin with more water or lemon juice before serving. Dice the whole cauliflower to florets also 1 day ahead to save time. I recommend roasting them on the same day.