Prepare for Baking: Adjust your oven rack to the middle position and heat the oven to 340°F (170°C). Lightly grease your baking dish if prone to sticking.
Brown the Beef: In a large skillet, cook the ground beef over medium-high heat, breaking it apart with a spoon. Continue cooking for approximately 6-8 minutes, or until it is thoroughly browned. Drain off any excess fat from the skillet.
Combine Meats and Aromatics: To the skillet with the cooked beef, add the minced garlic and cook for another minute until fragrant, stirring continuously. Remove from heat. Stir in the chopped cooked bacon and fresh parsley until evenly distributed. Transfer this mixture to your prepared baking dish, spreading it in an even layer.
Create the Egg Custard: In a medium bowl, vigorously whisk together the large eggs and heavy whipping cream until smooth. Whisk in the Dijon mustard, salt, and ground black pepper. Fold in approximately two-thirds (6 ounces / 170g) of the shredded Cheddar cheese into the egg mixture.
Assemble and Bake: Gently pour the egg and cheese custard evenly over the beef and bacon layer in the baking dish. Sprinkle the remaining one-third (3 ounces / 85g) of Cheddar cheese uniformly over the top.
Bake to Perfection: Place the casserole in the preheated oven. Bake for 35 to 40 minutes, or until the top is golden brown, the edges are bubbling, and a knife inserted near the center comes out clean.
Rest and Serve: Carefully remove the casserole from the oven and let it rest for at least 5-10 minutes before slicing and serving. This allows the bake to set further, making it easier to cut.