Place peppers directly over a gas-stove burner on high heat. As each section turns puffy and black, turn with tongs to prevent overcooking. (Or place peppers on a baking pan; broil in oven, turning as each side becomes charred.) Transfer to a large bowl; cover with plastic wrap. Let sweat until cool enough to handle, about 15 minutes. Transfer to work surface. Peel off blackened skin; discard. Halve; discard seeds. Roughly chop flesh; set aside.Advertisement
Heat oil in a large saucepan over medium-low heat. Add onion, garlic, and pepper flakes. Cook until onion is tender, about 8 minutes. Add tomatoes, tomato sauce, stock, oregano, and peppers. Bring to a gentle simmer; cook until slightly thickened, 15 to 20 minutes. Season with salt and pepper; serve. The sauce can be made ahead and refrigerated for up to 2 days, or frozen up to 1 month.