Tortellini Mac and Cheese

Bring a large pot of salted water to a boil over medium-high. Add shell pasta; return to a boil. Cook, stirring occasionally, 9 minutes. Add tortellini. Cook, stirring occasionally, until shells and tortellini are al dente, about 3 minutes. Drain; set aside.

While pasta cooks, melt 4 tablespoons of the butter in a broiler-safe Dutch oven over medium-high. Cook, stirring occasionally, until butter smells nutty, about 2 minutes. Add flour. Cook, whisking constantly, until toasted and light brown, about 2 minutes. Reduce heat to medium; gradually whisk in milk. Cook over medium, stirring occasionally, until mixture can coat the back of a wooden spoon, 5 minutes. Add cream cheese, mustard, salt, and pepper. Cook, whisking constantly, until smooth, 1 minute. Gradually add Cheddar and fontina, whisking between additions, until smooth, 1 minute. Remove from heat. Stir drained pastas into sauce in Dutch oven; toss to fully coat. Set aside.

Preheat oven to broil with rack about 8 inches from heat source. Microwave remaining 1 tablespoon butter in a small microwavable bowl on MEDIUM (50% power) until melted, about 30 seconds. Add panko to melted butter; toss to coat. Sprinkle evenly over pasta mixture. Broil until golden brown and crispy, about 3 minutes, checking often to prevent burning. Top with chives before serving.