Step 1

Melt butter in a large Dutch oven over medium. Add onion; cook, stirring occasionally, until tender and golden brown, about 5 minutes. Add tomatoes and their juices; bring to a boil over medium. Reduce heat to medium-low; simmer, stirring occasionally, until flavors meld and sauce thickens, about 30 minutes.

Step 2

Meanwhile, cook pasta in salted water according to package directions for al dente. Drain pasta, reserving 1/2 cup pasta cooking liquid.

Step 3

Stir cream, salt, black pepper, and crushed red pepper into tomato sauce. Add cooked pasta, and toss to coat. Add reserved pasta cooking liquid, 1/4 cup at a time, until desired consistency is reached.

Step 4

Divide pasta evenly among 4 bowls; sprinkle with Parmesan, and garnish with parsley.