Preheat the oven to 225°. Add the green beans to a large pot of boiling, salted water for 3 to 4 minutes. Add them to a large container of ice water and chill until cold. Strain and set aside.
In a large frying pan or cast-iron skillet over medium heat, add in the bacon and cook until very crispy brown, about 5 minutes. Remove the crispy brown bacon and set them aside.
Add the mushrooms and onions to the pan with rendered bacon fat over medium-high heat and brown them well, which takes about 10 to 15 minutes. Stir often.
Stir in the garlic and cook for 1 minute and then sprinkle in the flour and mix until completely combined. Pour in the chicken stock and cook over medium heat for 3 to 4 minutes or until very thick.
Pour in the cream and cook until it becomes very thick like alfredo sauce. Stir in the crispy bacon lardons, thyme, season with salt and pepper, and then fold in the green beans.