Heat oven to 400°F. Spray 18x13-inch rimmed baking sheet with cooking spray. In 12-inch skillet, melt butter over medium-high heat. Add cauliflower, onion, garlic, 1/4 teaspoon salt and the pepper. Cook 4 to 5 minutes, stirring frequently, until cauliflower begins to brown on edges and is tender when pierced with knife.