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1
Preheat the oven to 350.
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2
In a medium skillet, melt the butter in 2 tablespoons of the olive oil.
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3
Add the garlic and cook over moderately high heat until fragrant, about 30 seconds.
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4
Add the mushrooms and cook, stirring often, until softened, about 2 minutes.
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5
Add the onion and cook until softened, about 5 minutes.
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6
Transfer the mixture to a large bowl and add the spaghetti in tomato sauce, prosciutto, cream and 1/4 cup of the Parmesan.
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7
Season with salt and pepper.
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8
Add the eggs and stir to combine.
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9
Heat the remaining 2 tablespoons of olive oil in a large ovenproof skillet until shimmering.
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10
Swirl the pan to coat the sides with oil and pour in the frittata mixture, spreading it evenly.
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11
Cook over moderately high heat until the frittata is barely set on the bottom, about 3 minutes.
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12
Sprinkle the remaining 1/4 cup of Parmesan evenly over the top.
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13
Transfer the skillet to the oven and bake the frittata for about 30 minutes, or until it is firm in the center and cooked through.
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14
Run a knife around the edge to loosen the frittata.
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15
Using 2 oven mitts, invert a large round platter over the frittata.
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16
Flip the skillet and platter to unmold the frittata and serve it cut into wedges, hot or at room temperature.
