1. Set oven to 425°F. In a large sauce pot over medium-high heat, melt 3 tablespoons of butter and add the ground beef. Break beef into small crumbles and cook until browned. Once cooked, place beef in a small bowl and set aside.
2. Melt remaining butter and whisk in the flour to form a roux- cook for 1 minute. Whisk in the milk, ½ cup at a time, allowing it to thicken before adding the remaining milk. Once all the milk has been added, add the Velveeta and stir until melted. Turn off the heat, add the cheddar, and stir until melted. 3. Cook pasta to package instructions. Stir the pasta, peas and the beef into the cheese sauce. Mix well and place in a 9x13" casserole dish.
4. In a small bowl, combine the bread crumbs and rosemary and season with salt and pepper. Top the mac ‘n’ cheese with the bread crumbs and bake for 25 minutes, or until golden and bubbly. Remove from the oven and top with more fresh rosemary (if desired). Serve immediately.