1. Preheat oven to 325 degrees.

2. Bring a large pot of water to a boil; prepare an ice-water bath and set aside.

3. Using a sharp knife, cut an X in the bottom of each tomato. Add to boiling water and cook for 10 to 15 seconds; immediately transfer to ice-water bath. Let cool completely and drain. Gently peel and core tomatoes.

4. Place tomatoes in an even layer in a baking dish, core-side down. Season with salt, pepper, and crushed red pepper flakes. Drizzle with olive oil and top with basil sprig. Transfer to oven and cook for 45 minutes. Remove from oven and let cool slightly. Chop tomatoes and transfer to an airtight container; add enough olive oil to cover.