Bring a large saucepan of water to a boil over high heat, and blanch the beans for 3-4 minutes, or until they are bright green in colour and pliable but still very crunchy. Drain and run them under cold water. Place in a shallow, ovenproof casserole dish and pat dry with a paper towel.
In a small saucepan, melt the butter over medium heat. Add the garlic, mustard, salt, and brown sugar. Pour the mixture over the green beans, cover with plastic wrap, and refrigerate for at least 4 hours, or overnight.
Preheat the oven to 375. Gather 8-9 beans at a time and make bundles. Wrap with a slice of the (halved) bacon, and lie seam-side down in a greased baking sheet or casserole dish. Pour remaining marinade over the top.
Bake, uncovered, for 45 minutes, or until the bacon is cooked through and the beans look wrinkled. Cook under the broiler for an additional 1-2 minutes to crisp the bacon even more if desired. Serve warm or at room temperature.