Ingredients

For assembly:

• Kosher salt
• 12 ounces dried jumbo pasta shells
• 2 ounces low-moisture mozzarella cheese, shredded (about 1/2 cup)
• 1/2 ounce Parmesan cheese, finely grated (2 tablespoons)

For the filling:

• 24 ounces whole-milk ricotta cheese (about 2 2/3 cups)
• 10 ounces low-moisture mozzarella cheese, shredded (about 2 1/2 cups)
• 1/4 cup heavy cream
• 1 1/2 teaspoons finely grated garlic (2 to 3 large cloves)
• 1 1/2 teaspoons freshly ground black pepper
• 1 teaspoon kosher salt, plus more as needed
• 1/2 teaspoon dried Italian seasoning

For the sauce:

• 1 tablespoon olive oil
• 1 tablespoon oil from oil-packed sun-dried tomatoes
• 1 medium yellow onion, finely chopped (about 1 1/2 cups)
• 3/4 teaspoon kosher salt, divided, plus more as needed
• 1/4 teaspoon red pepper flakes (optional)
• 1 teaspoon dried Italian seasoning
• 5 ounces baby spinach (about 5 packed cups)
• 2/3 cup thinly sliced or julienned drained oil-packed sun-dried tomatoes (4 1/4 ounces)
• 2 teaspoons finely grated garlic (3 to 4 large cloves)
• 3 cups heavy cream
• 2 ounces Parmesan cheese, finely grated (2/3 packed cup freshly grated or 1/2 cup pre-grated)

1. Bring a large pot of heavily salted water to a boil. Add 12 ounces dried jumbo pasta shells and cook according to package directions (follow directions for shells that will be baked if specified). Drain well, then arrange on a baking sheet in a single layer.
2. Place 24 ounces ricotta cheese, 10 ounces shredded mozzarella cheese, 1/4 cup heavy cream, 1 1/2 teaspoons finely grated garlic, 1 1/2 teaspoons black pepper, and 1 teaspoon kosher salt in a large bowl. Use your fingertips to crush 1/2 teaspoon dried Italian seasoning into the bowl, then stir to combine. Taste and season with more kosher salt as needed.

Heat the oven and make the sauce:

1. Heat the oven to 375°F.
2. Heat 1 tablespoon olive oil and 1 tablespoon sun-dried tomato oil in a large skillet over medium heat until shimmering. Add 1 finely chopped medium yellow onion, 1/2 teaspoon of the kosher salt, and 1/4 teaspoon red pepper flakes if using. Use your fingertips to crush 1 teaspoon dried Italian seasoning over the onions. Cook, stirring occasionally, until the onions are softened and some pieces are just starting to brown around the edges, 5 to 8 minutes.
3. Stir in 5 ounces baby spinach a few handfuls at a time and cook, stirring constantly, until wilted, 1 to 2 minutes.
4. Stir in 2/3 cup thinly sliced drained sun-dried tomatoes, 2 teaspoons finely grated garlic, and the remaining 1/4 teaspoon kosher salt. Cook, stirring constantly, until the garlic is fragrant and just starting to stick and very lightly brown on the bottom of the skillet, 30 seconds to 1 minute.
5. Stir in 3 cups heavy cream and scrape up any browned bits from the bottom of the skillet. Bring to a lively simmer over medium-high heat. Reduce the heat to maintain a gentle simmer and simmer until very slightly reduced and the sauce coats the back of a spoon in a thin layer, about 1 minute.
6. Remove the skillet from the heat. Stir in 2 ounces grated Parmesan cheese until melted and combined. Taste and season with more kosher salt as needed.

Assemble and bake:

1. Spread about 1/2 of the sauce (about 2 cups) over the bottom of a 9x13-inch baking dish.
2. Stuff each pasta shell with about 2 heaping tablespoons of the filling and place filling-side up in the dish. You’ll have more shells than you need; this is in case any break or rip.
3. Spoon the remaining sauce evenly over the shells. Sprinkle with 2 ounces shredded mozzarella cheese and 1/2 ounce finely grated Parmesan cheese.
4. Bake uncovered until the sauce is bubbling around the edges and the cheese on top is melted, about 25 minutes. If desired and your baking dish is broiler-safe, you can place the shells under the broiler to lightly brown the cheese. Let cool slightly before serving, 5 to 10 minutes.