Blanch the green beans. Bring a large pot of water to a rapid boil. Add green beans, then cook for exactly 3 minutes. Immediately transfer to an ice bath to cool. While they cool, cook the almonds.
Toast the almonds. Melt 2 Tbsp unsalted butter in a skillet over medium heat. Add ⅓ cup sliced almonds, then toast, shaking the pan occasionally, until golden brown and fragrant, about 4-6 minutes. Using a slotted spoon, transfer the almonds to a small bowl.
Sauté shallot. To the same pan, add 2 Tbsp olive oil and warm over medium heat. Add 1 chopped shallot, season with ¼ tsp kosher salt, then cook, stirring occasionally, until softened, about 3-4 minutes. Add 1 clove chopped garlic, then cook 1 minute more.
Cook the French beans. Drain the green beans. Increase the heat to medium-high, then add the green beans to the skillet. Season with remaining 1 tsp kosher salt and ¼ tsp black pepper, then cook, stirring occasionally, until tender, about 5-7 minutes.
Flavor the green beans. When the green beans are tender, stir in 2 tsp fresh lemon zest and 2 Tbsp fresh lemon juice (about half a lemon). Stir well, then taste and adjust seasoning as needed.
Garnish, then serve! Transfer to a serving plate, then sprinkle the toasted almonds on top. Finish with a pinch of flaky sea salt. Enjoy!