Cook the cauliflower florets in a large pot of boiling salted water. Simmer for 15 minutes until the cauliflower is soft and cooked through. Drain in a colander and let cool (I like to make this the day before & keep in the fridge).
Preheat the oven to 400°F.
Brush the sides of a 9-inch springform or loose-bottom cake pan with melted butter. Mix the sesame and nigella seeds and sprinkle them around the inside of the pan so that they stick to the sides.
Take your peeled red onion and cut a few thin round slices off one end of the onion. Set aside, these will go on top of your cake.
Heat a small skillet and add the olive oil. Chop the rest of the onion and place in the pan with the rosemary. Cook for 10 minutes over medium heat, stirring from time to time, until soft. Remove from the heat and set aside to cool.
In a stand mixer or with an electric whisk (or your own elbow grease), beat the eggs until light and fluffy. This will take a couple minutes.
Add the chopped basil, flour, baking powder, salt, pepper and turmeric, and mix gently, just until combined.
Stir in the onion mixture, roasted brussels sprout rubble, parmesan, goat cheese gently by hand.
Last, add the cooled cauliflower florets and mix lightly to to thoroughly coat. Be careful not to break up the florets too much.
Pour the cake mixture into the pan, spreading it evenly, and arrange the reserved onion rings on top in a pattern.
Place in the center of the oven and bake for 30-40 minutes, until golden brown and set. A knife inserted into the center of the cake should come out pretty much clean, but baking time can vary based on your oven and pan, so keep an eye on it starting at 30 minutes.
Remove from the oven and let sit for at least 20 minutes before serving. It needs to be served just warm, rather than hot, or at room temperature.