1. Preheat oven to 350°F (180°C) degrees.
2. Break cauliflower into medium florets (this will cause less mess than chopping it).
3. Place floret in a pot with a teaspoon of salt, cover them with water and simmer for 15 to 20 minutes, until quite soft. Strain and let drip in the colander for a few minutes so they dry and cool.
4, While the cauliflower is cooling, prepare the batter. Halve the red onion and cut a few thin rings off the end of one side; set them aside. Coarsely chop the remainder of your onion.
5. Heat all of the olive oil in a saucepan and saute the chopped red onion and rosemary together until soft, about 8 minutes. Remove from heat and allow to cool.
6. Whisk eggs and olive oil and onion mixture together. Whisk flour, baking powder, turmeric, cheese, salt and black pepper together in a separate bowl and add to egg mixture, whisking to remove lumps. Stir in cauliflower gently, so most pieces remain intact.
7. Line the bottom of a 9-inch (24 cm) round springform pan with parchment paper. Butter the sides generously.
8. Put the sesame seeds in the pan and toss them around so that they stick to the sides. Pour in the cauliflower batter, arrange the reserved onion rings on top and bake cake in the center of the oven for 50 minutes, until golden brown and set.
9. Serve warm or at room temperature. Before you serve the cake, be sure to run a knife around the pan.