Add 2 tsp. salt, a splash EVOO and 20 jumbo pasta shells to a large pot of boiling water. Cook according to package directions. Drain and rinse until cool. Cover with damp paper towels.
1 inch water to a boil in the same pot. Add half of 1 lb. baby spinach. Let wilt before adding the rest; stir to wilt. Rinse with cold water; squeeze to get the spinach as dry as possible.
Place the spinach on a cutting board and chop with a bench scraper. (Or, if you dont have a bench scraper, let an adult chop with a chefs knife.)
Preheat the oven to 400 degrees . In a skillet, melt 2 tbsp. butter. Add 1 large onion, chopped; add 3/4 tsp. salt and 1/2 tsp. pepper. Cook over medium-high heat until tender, 6 to 7 minutes.
Add 1 lb. extra-lean ground beef; cook, breaking up with a wooden spoon, until no longer pink, about 3 minutes.
Remove the skillet from the heat and stir in the chopped spinach until well mixed. Stir in 1/3 cup shredded cheddar. Let cool.
Heat 2 cups milk in a microwave. In a medium saucepan, melt 2 tbsp. butter. Whisk in 1/4 cup flour; cook 1 minute. Whisk in 1/2 cup hot milk, then the rest. When it boils, whisk 1 minute.
Remove from the heat; season with 3/4 tsp. salt and 1/4 tsp. pepper. Whisk in 3/4 cup pure pumpkin puree. Stir in 1 cup shredded cheddar. Cover to prevent a skin from forming.
Hold a pasta shell in one hand and stuff with a large spoonful of the beef-spinach mixture (about a 1-inch ball). Cover the stuffed shells with a damp towel as you work.
Grease a 9-by-13-inch baking dish. Smear the bottom with 1/2 cup of the pumpkin sauce. Arrange the stuffed shells in the prepared pan with the openings face-down.
Spoon more sauce on top. Cover the dish with foil and bake until the sauce is bubbling hot, about 25 minutes. Let cool for 10 minutes before decorating with parsley leaves.