Preheat the oven to 400° F
Line the base and sides of a 9-inch spring-form cake pan with parchment paper.
Brush the sides and bottom with a thin layer of olive oil (~ 1 - 2 Tbsp worth)
Combine the sesame and nigella seeds and toss them around the inside of the pan; the idea being that they'll stick to the sides.
Prepare the Cauliflower
Place the cauliflower florets in a medium-sized saucepan and add 1 tsp sea salt.
Cover the cauliflower with water and bring the contents of the saucepan to a simmer over medium heat.
Allow to simmer for 10-15 minutes, or until the florets are quite soft. (You'll know they're done when they break apart when pressed with a spoon)
Drain the cauliflower and set aside in a colander to cool
Prepare the Cake
From one end of the onion, cut 4-6 round slices (each ~ ¼" thick), and set aside
Coarsely chop the rest of the onion, placing it, along with a Tbsp of olive oil in a small saute pan over medium heat. Sprinkle the rosemary over the top.
Cook for ~ 10 minutes, stirring from time to time until they're soft and fragrant.
Remove the onions from the heat and set them aside to cool.
Transfer the onions to a large bowl, adding the eggs and basil, and whisk well.
Add the flour, baking powder, turmeric, Parmesan, the other 1 tsp sea salt, and plenty of pepper. Whisk until very smooth.
Add the cauliflower, stirring gently, so as not to break up the florets
Bake and Serve
Pour the cauliflower mixture into the prepared pan, making sure it's spread evenly.
Arrange the reserved onion rings on top.
Place the cauliflower cake into the center of the oven and bake for 45 minutes, or until it's golden brown and set (a knife or toothpick inserted into the center of the cake should come out clean)
Remove from the oven and allow to rest for 20-30 minutes before serving.
It's best served just warm, rather than hot, or at room temperature.