Let's Cook

1. Step 1.

   Heat oil in large saucepan over medium heat until shimmering. Add onion, salt, and pepper and cook, stirring occasionally, until softened and lightly browned, about 10 minutes.

2. Step 2.

   Stir in garlic and pepper flakes and cook until fragrant, about 30 seconds. Stir in tomato puree, water, and sugar and bring to simmer. Reduce heat to medium-low and cook until flavors have melded, about 5 minutes.

3. Step 3.

   Stir all filling ingredients in bowl until thoroughly combined. Transfer filling to pastry bag or large zipper-lock bag (if using zipper-lock bag, cut 1 inch off 1 corner of bag).

4. Step 4.

   Adjust oven rack to middle position and heat oven to 400 degrees. Place shells open side up on counter. Pipe filling into shells until each is about three-quarters full. Divide remaining filling evenly among shells.

5. Step 5.

   Spread 1 cup sauce over bottom of 13 by 9-inch baking dish. Transfer shells, open side up, to prepared dish. Pour remaining sauce evenly over shells to completely cover.

6. Step 6.

   Cover dish tightly with aluminum foil and set on rimmed baking sheet. Bake until shells are tender and sauce is boiling rapidly, about 45 minutes. Remove dish from oven and discard foil; sprinkle fontina over top. Bake, uncovered, until fontina is lightly browned, about 15 minutes. Let shells cool for 25 minutes. Sprinkle with basil. Serve.