Prepare an ice bath so it’s ready when the green beans are cooked. (Ice bath how and why: To prepare your ice bath, add ice and cold water to a large bowl, leaving room to add the beans. The ice bath will stop the cooking and preserve the bright green color of the beans. You may skip this step. Simply account for some carryover cooking and a slightly less vibrant hue.) Set aside.
Bring a large pot of water to a boil. Add the green beans and boil for 3 to 4 minutes, give or take a minute depending on thickness of beans, or until crisp-tender or cooked to your preferred level of doneness. (Taste one to be sure.)
Drain the green beans and immediately place into the prepared ice bath. Once chilled, drain the beans very well and pat dry. (Prep-ahead tip: At this point, you may transfer the beans to a covered bowl or zip-top bag and refrigerate up to overnight.)
Meanwhile, preheat a large (12-inch works well) skillet over medium-low heat. Add the slivered almonds and cook, shaking the pan every 30 seconds or so, until the almonds are fragrant and lightly golden. As soon as they reach this point, transfer them to a plate.
In the same skillet, melt the butter over medium heat. Add the shallot and garlic and sauté until softened, about 2 to 3 minutes. While these are sauteing, I zest the lemon into the pan so it infuses the butter sauce.
Add the green beans, lemon juice, salt and pepper. Toss to incorporate, and then cook a few minutes more to warm through. Add the toasted almonds. Gently toss so that they become coated with the sauce, too.
Transfer the mixture to a serving platter. Some of the almonds will likely settle to the bottom of the pan, so scrape those over the beans. You’ll want to eat every last one!