Preheat oven to 350 degrees F.
Blanch green beans: In a large pot, bring water a boil, enough to fully cover the beans. Add the green beans (trimmed and cut in half) and blanch for 5 minutes. Once crisp tender, immediately transfer them to a bowl with ice and water for a few minutes until cold. Drain, pat dry, set aside. Note: An ice bath shocks the beans and immediately stops the cooking process, which keeps them vibrant green.
Cook bacon: In a deep skillet or pan, cook the bacon. Dice into small pieces. Set aside. Reserve the bacon grease for later.
Start the creamy sauce: In a saucepan, heat 2 tablespoons of the bacon fat, then sauté onions, garlic, and mushrooms for about 3 minutes until fragrant. Then melt 3 tablespoons of butter and sprinkle 5 tablespoons flour, stirring to coat, another 2-3 minutes to cook off the raw flour taste.
Finish the creamy sauce: Gradually, add chicken broth into the saucepan, stirring to break up any lumps. Next, stir in the milk. Stir until thickened - it will thicken as it heats up and comes to a low simmer. Add the soy sauce and umami mushroom seasoning, then season with salt and pepper to taste.
Assemble: In a 9x13 inch baking pan, toss the blanched green beans with the cooked bacon and the creamy sauce (make sure the sauce is pretty warm). Top generously with the panko breadcrumbs and crispy fried onions. Spray with cooking oil - this will help the onions turn golden brown in the oven.
Bake: Bake for 20-25 minutes, uncovered, until golden brown and bubbly. Enjoy!