Preheat the oven to 500 degrees F (260 degrees C).
Melt butter in a skillet over medium heat, whisking to ensure it doesn't burn, until light brown and nutty-smelling. Remove from heat.
Toss cauliflower florets with the brown butter and salt until well combined. Transfer to a baking sheet.
Roast in the preheated oven until just tender, about 15 minutes. Remove and toss hot cauliflower with red onion, Pecorino Romano cheese, and mint. Drizzle with lemon juice and season with more salt to taste.