Preheat the oven to 425 degrees F. In a small bowl, combine the onion, red wine vinegar, a pinch of salt, and 2 tablespoons warm water. Set aside to pickle until serving.
On a baking sheet, toss the cauliflower with 1 tablespoon olive oil and season with salt and pepper. Roast until golden, tossing halfway through, about 18 minutes.
Meanwhile, in a medium bowl, whisk the eggs and milk with ¼ teaspoon salt and ¼ teaspoon pepper.
Melt the butter in an 8-inch cast iron pan or oven-safe nonstick skillet over medium-high heat. Add the chickpeas and most of the scallions (reserving a small amount for garnish) and cook, stirring, until both are lightly golden in spots, about 4 minutes. If the cauliflower has not yet roasted fully, set pan aside off-heat until cauliflower is finished.
Then, add the cauliflower, most of the herbs (reserving a small amount for garnish), and egg mixture to the pan over medium-high heat. Cook, stirring with a spatula to create large curds, until eggs are wet on top but otherwise set, 1 to 2 minutes.
Transfer pan to oven and bake until frittata is puffed up and the eggs are just set on top, about 8 minutes. Let sit 5 minutes before serving.
When ready to serve, drain the red onion. Top the frittata with Parmesan, reserved herbs and scallions, and pickled red onion.