Pour 2 cups of chicken broth and half of the drained broccoli into a food processor and puree.
Melt 1 tablespoon butter in a large pot; add onions and saute over medium heat until onions begin to soften, about 6 minutes. Add remaining 2 cups chicken broth. Bring to a simmer.
Pour chicken broth/broccoli mixture into the pot. Simmer over medium-high heat 3 to 4 minutes, stirring occasionally.
Place cheese packet from Horizon(R) Pasta Shells & White Cheddar Cheese, flour, melted butter, and half and half in a bowl. Stir until blended. Slowly add mixture to large pot; whisk over medium-high heat 2 minutes.
Add Horizon(R) Pasta Shells & White Cheddar Cheese and remaining broccoli. Add a pinch or two of sea salt. Simmer until macaroni is done, 10 to 12 minutes.
Remove from heat. Serve and top with shredded Cheddar cheese.