Heat 2 tbsp olive oil in a large skillet over medium-high heat. Add cauliflower florets and cook 8–10 minutes, stirring occasionally, until browned in spots.
Remove cauliflower and set aside.
Add remaining oil to the skillet along with onions, cooking 12–15 minutes, until deeply caramelized. Add garlic in the last minute.
Return cauliflower to the skillet and toss to combine.