Heat 2 tablespoons olive oil in a large skillet over medium heat until shimmering. Add 1 medium onion (thinly sliced) and cook, stirring occasionally, about 5 minutes until softened and translucent.
Add 1/2 head cauliflower (cut into bite-sized florets) and 16 ounces mushrooms (cleaned and quartered) to the skillet and stir to combine.
Sprinkle 1/2 teaspoon Italian seasoning, 1/2 teaspoon dried oregano, and salt and black pepper to taste over the vegetables; stir to coat evenly.
Pour in 1/4 cup vegetable broth and stir. Bring to a gentle simmer and cook, stirring occasionally, until the cauliflower is tender and the vegetables are lightly browned, about 10 minutes.
Stir in 3 cloves garlic (minced) and red pepper flakes to taste, if using; cook for about 30 seconds more until the garlic is fragrant.
Remove the skillet from the heat, sprinkle with chopped fresh parsley for garnish, and serve immediately.