Warm the olive oil in a 12-inch skillet over medium-high.
Add the onion, season generously with salt and cook, stirring, until the onion is translucent, soft and starting to turn golden; 7 or 8 minutes; if necessary, decrease the heat to medium to prevent scorching.
Add the garlic and cook until fragrant and softened; about 2 minutes.
Stir in the oregano, red-pepper flakes and fennel seeds; add the Calabrian chiles, roasted red peppers and tomatoes; crush the tomatoes by hand, if using whole.
Season with salt and pepper; bring to a simmer, adjust the heat to maintain a simmer and cook for about 5 minutes to slightly reduce the sauce and blend the flavors.
Add the shrimp, scallions and capers; cook until the shrimp are curled, pink and opaque; 3 to 7 minutes, depending on size and quantity.
Turn off the heat and taste the sauce.
Add more red-pepper flakes, salt and pepper, if you like.
Sprinkle the cheese over the top and serve, passing more Parmesan and red-pepper flakes at the table.
