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1
Heat the oven to 375 degrees F and arrange a rack in the middle.
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2
Bring a large pot of heavily salted water to a boil over high heat.
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3
Add the pasta shells, stir, and cook until al dente, about 10 minutes.
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4
Place a colander in the sink and drain the shells.
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5
Rinse with cold water until cool and then drain well.
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6
Transfer the shells to a medium bowl and set aside.
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7
Return the colander to the sink.
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8
Heat the oil in a large frying pan over medium heat until shimmering.
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9
Add the onions, garlic, oregano, 1/2 teaspoon of the salt, and the measured pepper and cook, stirring often, until the onions have softened, about 5 minutes.
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10
Add the chard and cook, tossing constantly with tongs, until completely wilted and tender, about 5 minutes.
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11
Transfer the mixture to the colander and let sit, stirring occasionally, until cool enough to handle, about 10 minutes.
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12
Meanwhile, place the ricotta, 1/4 cup of the Parmesan, the egg, and the remaining 1/2 teaspoon salt in a large bowl and stir to combine; set aside.
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13
Place the marinara or tomato sauce and water in a medium bowl and stir to combine; set aside.
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14
When the chard mixture is ready, squeeze large handfuls to release any excess liquid and transfer to a cutting board.
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15
Finely chop and add to the ricotta mixture.
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16
Stir until evenly combined.
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17
Taste and season with salt and pepper as needed.
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18
Transfer the ricotta-chard mixture to a large resealable plastic bag and snip 1/2 inch off one bottom corner.
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19
Evenly spread 1 cup of the tomato sauce mixture on the bottom of a 13-by-9-inch baking dish.
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20
Pipe about 2 tablespoons of the ricotta mixture into each pasta shell and place in a single layer, open side up, in the baking dish.
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21
(Alternatively, use a spoon to fill the shells.)
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22
Pour the remaining tomato sauce mixture evenly over the shells and sprinkle with the remaining 1/2 cup Parmesan.
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23
Cover the dish with foil and bake until the sauce just starts to bubble around the edges, about 20 minutes.
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24
Remove the foil and continue baking until the sauce is bubbling vigorously and the edges of the pan have started to brown, about 10 minutes more.
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25
Remove from the oven and let cool 5 minutes before serving.
