1) Heat broiler, with rack in top position. In a medium saucepan, cook bacon over medium until browned and crisp, about 10 minutes. With a slotted spoon, transfer to a paper towel to drain. Add onion and garlic to pan and season with salt and pepper. Cook, stirring occasionally, until onion is soft, about 8 minutes. Add flour and cook, stirring, 30 seconds.

2) Slowly add milk, whisking constantly. Add tortellini and bring to a boil over medium-high, stirring occasionally. Reduce to a simmer; cook, stirring often, until liquid has thickened, about 4 minutes. Remove from heat; stir in bacon and 1/4 cup Parmesan. Transfer to a 2-quart broilerproof baking dish and top with 1/4 cup Parmesan. Broil until top is golden brown, 3 to 5 minutes.