Step 1Melt 4 tablespoons of the butter in a saucepan over medium-high heat. Add the onion and cook until softened but not browned, about 4 to 5 minutes. Add the flour and cook 2 to 3 minutes. Add the milk, stirring constantly. When it comes to a boil, reduce the heat and simmer for about 15 minutes, stirring frequently. Add the Cheddar cheese, stirring to melt. Don't let it come to a boil, or the sauce will separate. Season to taste with salt and pepper.
Step 2Meanwhile, melt the remaining tablespoon of butter in the microwave and mix with the bread crumbs and Parmesan cheese. Set aside.
Step 3Heat the broiler to high.
Step 4Cook the farfalle and broccoli in boiling salted water until the pasta is al dente and the broccoli is tender, 10 minutes. Drain.
Step 5Mix the pasta, broccoli and sauce together and turn into a 13x9-inch baking dish. Top with the bread crumb mixture and place the dish about 6 inches beneath the broiler until toasted and golden brown, 30 to 60 seconds. Watch carefully to prevent burning.