Bring a large pot of water to boil and preheat your oven to 400F. Wash the cauliflower and trim its bottom stem.
When the water is boiling, transfer the cauliflower to the pot. Reduce the heat to medium low. Simmer the cauliflower for 10-12 minutes, or until the center of the cauliflower can be pierced with a knife (it's OK if the knife doesn't pierce very easily—the cauliflower should be cooked through, but not super soft).
When the cauliflower is ready, remove the pot from heat. Drain the water and remove the cauliflower. Transfer the cauliflower to a surface that's been lined with a tea towel or hand towel. Carefully (the cauliflower will be hot) use another tea towel to pat the cauliflower dry. You want to dry the surface of the cauliflower as much as possible.
Whisk the sauce ingredients together in a small bowl. You should end up with about 1 1/4 cups tahini sauce.
Line a baking sheet with parchment. Transfer the cauliflower to the baking sheet. Use a basting brush to cover the whole surface of the cauliflower with the lemon tahini sauce. Be generous! The cauliflower should be well coated.
Transfer the baking sheet with the cauliflower to the oven. Bake the cauliflower for 10 minutes, then remove it from the oven and baste it with more sauce. Return the cauliflower to the oven and bake it for another 15-20 minutes, or until the sauce has darkened and the bottom of the cauliflower is starting to brown.
Cut cauliflower into six wedges. Serve it with the remaining tahini sauce. If you have tahini sauce left over, you can use it for salads, bowls, or as a dip. The sauce will keep for up to 5 days in the fridge.
Notes
Leftover cauliflower will keep in an airtight container in the fridge for up to three days.