Place the cauliflower florets and one teaspoon of the salt in a medium (3-quart) saucepan, and add water to cover. Bring up to a boil over medium-high heat, then turn down to low and let simmer until soft but not falling apart, about 15 minutes.

Drain the cauliflower in a colander and set aside to cool.

Peel the onion and trim off the dry stem end. Cut off four 1/4-inch thick slices of onion rings from the stem end and set aside. Dice the remaining onion.

Heat the olive oil, diced onions, and chopped rosemary in a small (8-inch) skillet over medium heat. Cook for ten minutes, stirring often, until the onions are just beginning to brown. Remove from the heat and set aside.

Cut out a 9-inch circle and two 15-by-4-inch strips of parchment paper.

Open the latch of a 9-inch springform pan to expand the outer band. Place the parchment circle in the bottom of the pan, and arrange the strips of parchment along the inside of the outer band, overlapping them slightly. Close the latch of the pan, securing the parchment strips. Don’t worry if they curl in a bit — when you brush them with melted butter, it will tack the paper down neatly.

In a small bowl, microwave the butter just until melted, about 20 seconds. Use a pastry brush to apply it to the parchment paper in the pan, covering the bottom and sides.

In another small bowl, stir together the sesame and nigella seeds. Hold the lined, greased pan over the sink, and sprinkle the seed mixture evenly onto the parchment strips on the sides of the pan.

In a large mixing bowl, whisk together the sautéed onion mixture, eggs, cheese and basil until the yolks and whites are thoroughly blended. Whisk in the flour, baking powder, the remaining teaspoon of salt, pepper, and turmeric. Finally, use a spatula to gently fold in the cauliflower florets, taking care not to break them up in the batter.

Pour the batter into the lined baking pan, spreading it out evenly with the spatula. Tap the pan firmly against the counter top to remove any air bubbles.

Arrange the rings of red onion on top of the cake batter — they will sit on top, sinking in just a bit as the cake cooks.

Bake the cake until golden brown on top and fully cooked through, for 45 minutes or until an instant-read thermometer registers 195ºF in the center of the cake.

Allow the cake to cool for at least 30 minutes. Use a sharp knife to slice into wedges and serve warm or at room temperature.