Coat a 9x13 inch baking dish with cooking spray and set aside.
In a large pot over medium-low heat, cook the bacon until crisp. Transfer the bacon to a plate and set aside.
Add the onions to the pot and cook until softened, approximately 3-4 minutes. Stir in the garlic and cook for 1 additional minute.
Sprinkle the flour over the onions, stirring to combine. (It will look a little like a paste.) Continue cooking, stirring constantly with a wooden spoon, until golden brown, about 4 minutes.
Slowly add the milk and cream, whisking constantly to incorporate.
Stir in the nutmeg. Increase the heat to medium-high and bring the sauce to a low boil, whisking constantly.
Reduce to a simmer, whisking occasionally, and cook until the sauce sauce is thick and coats the back of a spoon, about 4-5 minutes.
Stir in the salt, pepper, mustard powder and peas.
Add the cheeses (except the parmesan), 1 cup at a time, whisking until each addition is completely melted before adding more. Remove from heat.
Stir tortellini into the sauce and toss to coat.
Transfer tortellini mixture to the prepared baking dish and place on a rimmed baking sheet covered with foil. Sprinkle parmesan evenly over pasta.
Bake until golden and bubbling, approximately 20-25 minutes.
Crumble the bacon and top the pasta with the bacon.
Allow to cool 5 minutes before serving.
Notes
Tortellini al Forno can be assembled, covered and refrigerated for up to 48 hours before baking.You can also FREEZE your Tortellini al Forno covered tightly for up to 2 months. To cook your frozen tortellini al forno, unwrap and cover lightly with aluminum foil. Bake for 1 hour or until cooked through, bubbling and golden brown. Uncover and bake for an additional 10 minutes to brown the top even more.