Lightly oil the bottom of a heavy sauce pan. Over medium heat, cook the pancetta until the fat renders. Add the onion to the pan, and cook until softened and beginning to brown, approximately 10 minutes. Add the chopped garlic to the pan and cook until fragrant. Add the tomatoes and stir to combine. Rub the oregano between your palms into the pan, add the hot pepper flakes, and salt and pepper to taste. Add enough of the reserved juice to make it "saucy", lower the heat to low, and simmer, covered, for 40 minutes to 1 hour. Add more reserved juice or water as needed.

Cook pasta according to package directions. Drain, and toss at once with 1/2 cup of the sauce. The hot pasta will absorb the hot sauce and keep it from clumping together as it cools. Serve at once, passing the grated cheese to taste.

Note: Add a crisp green salad and some garlic bread, and you've got a minimal hands-on time weeknight supper on the table.