Bring a large pot of water to a boil and stir in two tablespoon salt. Add the green beans and blanch 5 minutes (start timing as soon as you add the beans). Meanwhile, prepare a large bowl of ice water to transfer the greens to. Using a slotted spoon, transfer beans to ice water to stop cooking. Drain the beans and set aside.
Melt 3 tablespoons butter with 2 tablespoons olive oil over medium heat in a Dutch oven or large pot. Add mushrooms and cook until nicely browned and starting to soften, 3-5 minutes. Season with 1 teaspoon salt and add garlic; sauté 30 seconds.
Whisk in flour then cook, while stirring for 1 minute. Turn heat to low then gradually whisk in chicken broth and heavy cream followed by Dijon, Worcestershire sauce and all spices/seasonings.
Bring sauce to a boil, whisking constantly then reduce heat to a simmer, whisking often until thickened, about 5 minutes.
Remove skillet from heat and whisk in cheddar until melted followed by Parmesan cheese until melted. Whisk in sour cream until blended.
Pat green beans dry then add to sauce; stir to combine. Stir in 1 ½ cups fried onions. Stir in additional broth if needed to thin.
Transfer green beans to a lightly greased 9x13 baking dish. Top with desired amount of onions. Bake at 350 degrees F for 15-20 minutes or until hot and bubbly.