Preheat your oven to broil. Lightly grease a 2-quart oven-safe baking dish with nonstick spray.
In a small bowl, stir together the panko, parmesan, parsley and 2 tablespoons olive oil. Set aside.
Bring a large pot of water to a boil. Prepare an ice bath (large bowl full of water and ice) and set out a clean kitchen towel. Boil green beans until just slightly tender, about 3-4 minutes. Place directly into the ice bath to stop the cooking process, then transfer to the kitchen towel. Pat dry.
Place 5 bacon in a large, rimmed saucepan. Turn heat to medium and let cook until slightly crispy, about 3 minutes per side. Remove from the pan, let cool, and chop into large pieces. Drain some of the bacon fat.
In the same pan, add diced onions and cook until slightly caramelized, about 8-10 minutes. Stir often. Add 1 tablespoon olive oil. Sprinkle the flour over top and continue stirring. Cook for 1-2 more minutes, just until all of the flour turns golden brown.
Slowly add the milk a few splashes at a time, stirring between each addition. Once all the milk has been added, increase heat to medium high. Stir in salt, pepper, and nutmeg. Continue cooking and stirring with a wooden spoon for 8-10 minutes, until sauce thickens and is a gravy-consistency. Make sure to scrape the bottom of the pan often to prevent any burnt bits. Remove from heat and stir in Greek yogurt.
Transfer half of the green beans and bacon to the baking dish. Spread half the sauce over top, and repeat. Finally, top with breadcrumb mixture.
Finally, just broil for 1-2 minutes (Watch CLOSELY, it cooks fast! I learned the hard way). Garnish with more fresh parsley to serve.