Steam or blanch your string beans until just under your desired softness (they will finish cooking in the oven). Drain and season with salt
In a large skillet, cook the bacon over medium low heat until crisp. Remove from the pan with a slotted spoon to drain on paper towels. Drain all but about 2 tablespoons of the bacon grease from the pan, then return the pan to medium heat.
Add the onions and cook, stirring occasionally, about 5 minutes or until translucent. stir in the mushrooms. Add the garlic and cook for 1 minute.
Stir in sour cream and cook until it thickens slightly.
Add the green beans to the mixture and salt and pepper to taste. Fold in the bacon until well combined.
Pour the mixture into a 9 inch baking pan (square or oblong)
For The Topping:
Add the breadcrumbs, parmesan cheese, olive oil, and pepper to a small bowl and mix to combine.
Sprinkle the mixture over the green bean mixture in the pan and bake, uncovered, for 25 minutes or until the topping is golden brown.
Serve hot.
Notes
Steaming the green beans before adding them to the casserole ensures they don’t turn out too tough. It’s an important step, don’t be tempted to skip this and rely on them cooking through in the oven.
Be sure to season your seamed green beans to avoid a bland dish.
To use frozen green beans, just thaw and drain them before adding them to the sauce,
To use canned green beans there’s no need to cook them first, just drain them well before using.
Don’t use anything less with less fat than half-and-half as it may thin out the sauce.
Some bacon can be salty, so go light on the salt in the sauce until the end, then taste and season as needed before adding the mixture to the baking dish.
If you have vegetarians for dinner simply omit the bacon and saute the mushrooms and garlic in olive oil or butter.