Baked Shells with Winter Squash

I made this for dinner the other night, and it was unique and delicious!

Butter for the dish
4 Tbsp. olive oil
2 onions, halved and thinly sliced
salt and pepper
2 tsp. chopped fresh rosemary ( I used 1 tsp. dried)
1 lb. small pasta shells (not jumbo and not the tiny ones)
1 box (12 oz.) squash puree, thawed
1 cup grated Parmesan
3 slices crusty baguette, cut into 1/4-inch cubes (1 1/2 cups)

1. Preheat oven to 400. Butter a 9x13inch baking pan. Heat 3 Tbsp of the oil in a large skillet over med-low heat. Add the onions; season with salt and pepper. Cover;cook until the onions are soft and release their liquid, about 15 minutes. Uncover;raise the heat to medium. Cook, stirring, until the onions are browned, 20-25 minutes. Stir in 1/2 of the rosemary.

2. Meanwhile, cook the pasta for 2 minutes less than the package instructions. Drain, reserving
1 1/2 cups of the cooking water. Return pasta to the pot.

3. Stir squash and pasta water into the onions; simmer for 2 minutes. Toss the squash mixture and 1/2 cup of the Parmesan with the pasta. Transfer to the prepared baking dish.

4. Combine bread cubes with remaining Parmesan, rosemary, and oil; season with salt and pepper. Top pasta with bread cubes; bake until golden brown, 10-15 minutes.