Whisk the cream, eggs and some salt and pepper in a large bowl.
Heat the butter and oil in a 10-inch nonstick ovenproof saute pan over high heat. Add the onions, squash and sage and saute until lightly browned and tender, about 8 minutes. Reduce the heat to medium and wait 2 minutes to allow the pan to come down to temperature. Pour the egg mixture over the squash and onions and cook until the eggs start to set, about 2 minutes. Sprinkle with the cheese and continue to cook until the frittata is almost set but the top is still runny, about 4 minutes.
Place the frittata under the broiler and broil until the top is golden brown, about 3 minutes. Let stand for 5 minutes. Using a rubber spatula, loosen the frittata from the pan and invert it onto a plate. Cut into wedges and serve.
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