Heat the oven to 375ºF. Bring a large pot of salted water to a boil. Add the shells to the boiling water and cook until just shy of al dente, about 10 minutes. Drain, and transfer shells to a large rimmed baking sheet. Drizzle with a little olive oil to keep them from sticking together.

Heat 2 tablespoons of oil from the jar of sun-dried tomatoes in a large skillet over medium. Once hot, add shallots and garlic; cook 3 to 5 minutes, until soft.Add broth; bring mixture to a simmer. Gently simmer for 5 to 6 minutes, until the liquid slightly reduces.Stir in half-and-half or heavy cream, ¼ cup Parmesan, Italian seasoning, ½ tsp. salt, ½ tsp. black pepper, and sun-dried tomatoes; gently simmer mixture for 4 to 5 minutes. (The sauce will be thin, however it will reduce while the shells bake.)

Place thawed spinach in a clean kitchen towel and squeeze over the sink to remove as much liquid as possible. Place spinach in a large mixing bowl along with ricotta, remaining ¼ cup Parmesan, lemon zest, and ¼ tsp. each of salt and black pepper; mix well to combine.Fill each cooked pasta shell with ricotta mixture.

Pour half of the sun-dried tomato sauce into a 3-quart baking dish (such as a 9x13). Arrange stuffed shells over the sauce, and spoon remaining sauce overtop. Scatter with mozzarella cheese.Cover the dish tightly with foil and bake for 15 minutes. Remove the foil and bake, uncovered, until the cheese is melted and the sauce is bubbling, about 15 more minutes. Let sit 5 minutes, then serve.