Blanch the green beans: Bring a medium pot of salted water to a boil. Add the trimmed green beans and cook for 3 to 5 minutes until they are bright green and crisp-tender. Immediately transfer the beans to an ice bath to stop the cooking process. Drain well and set aside.
Toast the almonds: Place the slivered almonds in a dry, small skillet over medium heat. Cook, stirring frequently, for 3 to 5 minutes until they are golden brown and fragrant. Remove the almonds from the skillet and set them aside.
Make the brown butter: Return the same skillet to medium heat. Add the 4 tablespoons of butter. Let the butter melt, then continue cooking. It will foam, then the milk solids will begin to brown and sink to the bottom, giving off a nutty aroma. Watch carefully to prevent burning. This takes about 3 to 5 minutes.
Remove the skillet from the heat immediately once the butter solids are deep brown. Stir in the lemon juice.
Combine: Add the blanched and drained green beans to the skillet with the brown butter. Toss gently to coat.
Stir in the toasted almonds. Season with salt and pepper to your taste. Serve immediately.
Notes
For the best texture, make sure the blanched green beans are completely dry before adding them to the brown butter.
You can substitute slivered almonds with sliced almonds if preferred.
If you prefer a lighter sauce, use half butter and half olive oil for the browning step.