1

In a large skillet over medium heat, warm oil. Add onion and garlic and cook, covered, stirring occasionally, until onion is golden, about 8 minutes.

2

Transfer onion mixture to a 6- or 7-qt slow cooker. Stir in tomato paste and 1 tsp salt until mixed well; top with tomatoes. Place basil sprigs in center of 12-inch piece of cheesecloth; gather ends together and tie with kitchen twine to form a bundle.

3

Push basil bundle into tomatoes. Cover and cook until tomatoes have released their juices and are tender, 2 1/2 to 3 hours on High or 5 to 6 hours on Low. Remove and discard basil bundle.

4

Puree tomato mixture with an immersion blender or let cool 5 minutes and puree in batches in a food processor. Cook, uncovered, until sauce is slightly thickened, 2 to 3 hours on High. Stir in 1/4 cup basil leaves, remaining 1/2 tsp salt, and pepper.

5

Meanwhile, cook penne according to package directions. Drain and cover to keep warm. To serve, divide pasta evenly among 6 large shallow bowls; top each serving with 1/2 cup tomato sauce and remaining 1/4 cup basil leaves. (Leftover tomato sauce can be refrigerated in an airtight container up to 5 days.)

6

Per serving: 1/2 cup tomato sauce and 1 cup pasta