Preheat the oven to 400 F.
Place the cauliflower and 1 tsp. of salt in a saucepan. Cover with water and bring to a simmer. Cook for about 15 minutes until florets are quite soft.
Drain and set aside in a colander.
Cut 3 round slices, each 1/4 inch thick, off one end of the red onion and set aside.
Coarsely chop the rest of the onion and place in a small pan with basil olive oil and rosemary. Cook gently for about 10 minutes.
Remove from heat and let cool.
In a large bowl, combine onion mixture, eggs, and chopped basil. Whisk well.
Add the flour, baking powder, 1 1/4 cups Romano cheese, 1 tsp salt, and black pepper. Whisk until smooth.
Fold in the cauliflower gently.
Line the base and sides of an 8 1/2 inch springform cake pan with parchment paper. Brush the sides with melted butter. Mix the sesame seeds and sprinkle along the sides.
Pour the cauliflower mixture into the pan and spread evenly. Arrange the reserved onion rings on top.
Bake in the center of the oven for 45 minutes until golden brown and set.
Let rest for about 15 minutes before serving. Sprinkle with the remaining shredded Romano cheese.
Serve warm.