Pre heat your oven to 350 degrees and heat a large frying pan over medium heat.
Cook the bacon until VERY golden brown and crispy on each side. Once cooked, transfer to a paper towel lined plate and blot off any excess fat. Discard most of the fat in the pan, reserving about 1 Tbsp.
Heat the pan to medium/high and add in the mushrooms and onion. Cook, stirring occasionally, until golden brown, about 3 minutes.
Add in all the other ingredients, except the beans and bring to a boil. Once boiling, cook, stirring constantly, until the mixture begins to thicken, about 4 minutes. Then, reduce the heat to medium and cook, stirring occasionally, until the mixture is VERY thick, like condensed soup, about 7-8 minutes.
While the sauce cooks, bring a large pot of salted water to a boil. Add in the green beans and cook until they just begin to become fork tender, about 7-8 minutes. Drain and pat as much of the water off of them with a paper towel.
Add the green beans into the sauce and stir until well coated. Press into an 8x8 inch pan and cook until bubbly, about 15-20 minutes.
While the beans cook, place the bacon in a food processor and pulse until small crumbs form. Sprinkle on the casserole and then bake for an additional 5 minutes.