Preheat oven to 375 degrees. In a large pot, cook the pasta in boiling salted water until al dente (about 5 minutes - don't overcook here!). During the last minute of cooking, add the chopped broccoli. Drain the shells/broccoli and set aside.
In a large, heavy-bottomed pot, melt the butter over medium heat.
When the butter has melted, add the flour and whisk together to form the roux. Keep whisking over medium until the color changes to straw-like and has an almost nutty smell. This will take about five minutes.
Slowly whisk in the milk. Whisk well to get rid of all clumps. Bring to a simmer and continue cooking/whisking until sauce thickens - about five more minutes.
When sauce has thickened, add the grated cheddar cheese and garlic salt. Add the cooked pasta and broccoli. Stir well to coat.
Spray a 9 x 13" pans with nonstick spray. Pour in the macaroni and cheese. Add the feta -- stirring very gently. Top with panko breadcrumbs and place in oven for 25 minutes.
Let macaroni and cheese cool slightly before serving. Enjoy!