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1
Preheat the oven to 375 degrees F.
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2
Heat the oil in a 10-inch nonstick skillet over medium heat.
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3
Slip in the onion and cook until softened, about 5 to 6 minutes.
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4
Push onion slices to one side of the skillet, and lay the tomato slices in one layer in the cleared space.
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5
Sear the tomato, turning once until the slices soften just at the edges, about 30 seconds per side.
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6
Remove the tomato to a plate, and let the onion continue to cook while you prepare the eggs.
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7
Beat the eggs with the salt in a bowl.
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8
Stir in the basil and 1/4 cup of the grated cheese until well mixed.
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9
Spread the onion slices in an even layer in the bottom of the skillet.
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10
Pour the eggs on top, and let cook over medium-low heat until the eggs begin to set around the edges of the pan, about 2 to 3 minutes.
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11
Arrange the tomato slices on top of the frittata and drop tablespoons of the ricotta between the tomato slices.
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12
Sprinkle all over with the remaining grated cheese.
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13
Bake the frittata until it is set all the way through and the top is golden, about 18 minutes.
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14
Let rest for a few minutes, then run a knife around the edge of the skillet and invert onto a plate or cutting board.
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15
Serve in wedges, warm or at room temperature.
