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1
Heat the oil in a large heavy-bottomed pot or Dutch oven over medium heat until shimmering.
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2
Add the onion, garlic, oregano, and red pepper flakes, season with salt and pepper, and cook, stirring occasionally, until the onion has softened, about 8 minutes.
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3
Increase the heat to medium high, add the sausage, and cook, breaking up the meat into smaller pieces with a wooden spoon, until its no longer pink and is starting to brown, about 6 to 8 minutes.
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4
Add the tomatoes, stir to combine, and bring the mixture to a boil.
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5
Reduce the heat to low and simmer, stirring occasionally, until the flavors meld and the sauce thickens slightly, about 30 minutes.
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6
Remove from the heat.
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7
Meanwhile, heat the oven and make the filling.
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8
Heat the oven to 375 degrees F and arrange a rack in the middle.
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9
Place all of the measured ingredients in a medium bowl, season with pepper, and stir to combine; set aside.
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10
Spread 1/2 cup of the sauce in a thin layer over the bottom of a 13-by-9-inch baking dish.
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11
Layer 3 of the noodles over the sauce.
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12
Top with another 1 1/2 cups of the sauce and spread it evenly to the edges of the dish.
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13
Using a spoon, dollop a third of the ricotta mixture evenly over the sauce and flatten the dollops with the back of the spoon (the mixture will spread out more as it cooks).
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14
Sprinkle evenly with a quarter of the mozzarella.
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15
Make 2 more layers of noodles, sauce, ricotta, and mozzarella.
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16
Finish with a layer of the remaining noodles, sauce, and mozzarella.
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17
Sprinkle with the Parmesan.
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18
Coat 1 side of a large piece of aluminum foil with olive oil and cover the dish tightly with the foil, oil-side down.
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19
(You can prepare the lasagna up to this point up to 1 day ahead and refrigerate; allow to come to room temperature before baking.)
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20
Bake until the sauce is starting to bubble around the edges, about 20 minutes.
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21
Remove the foil and continue baking until the top of the lasagna is browned, about 30 minutes more.
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22
Remove the pan to a wire rack and let cool for 15 minutes.
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23
Sprinkle with the parsley before slicing and serving.
