1Heat oven to 350°F. Spray 3-quart casserole with cooking spray.
2In large skillet, heat butter and oil over medium heat. Add onion; cook 2 to 3 minutes, stirring occasionally, until soft. Add cauliflower and 1/2 teaspoon of the salt. Cook 2 to 3 minutes, stirring occasionally, until cauliflower begins to brown. Spoon mixture into casserole.
3In medium bowl, beat Bisquick mix, milk, eggs and remaining 1/2 teaspoon salt with whisk. Stir in bacon and chives. Reserve 2 tablespoons of the cheese; stir remaining cheese into egg mixture. Pour evenly over cauliflower mixture. Sprinkle with reserved 2 tablespoons cheese.
4Bake uncovered 35 to 45 minutes or until knife inserted in center comes out clean.