Render the bacon in a large cast iron skillet on medium heat until crispy, about 5-7 minutes.
Blanch the green beans in boiling salted water until tender but still crisp, about 3 minutes. Transfer the green beans to an ice bath to stop the cooking and preserve the color. Drain once they are cooled.
Remove the bacon from the skillet, leaving 2 tablespoons bacon fat.
Add the chopped shallots and sweat until translucent, about 3 minutes. Add the garlic and cook for 1 minute.
Add the green beans, cooked bacon and toasted walnuts to the pan and warm through.
Finish with lemon juice or sherry vinegar and season with salt & pepper.