Start with a large skillet over medium heat. Add the almonds and cook, stirring frequently (careful, they burn quickly). After 3 minutes, reduce the heat to medium-low, and continue stirring until they are fragrant and turning golden on the edges, about 1 to 3 more minutes. Transfer the almonds to a bowl to cool.
Make sure the heat is set to medium-low and place the skillet back on the heat. Immediately add the green beans, water and ¼ teaspoon salt. Cover and cook, stirring occasionally, until the beans are nearly tender, about 7 to 10 minutes. Remove the lid, raise heat to medium-high, and cook until the liquid evaporates, 2 to 5 minutes. Remove the skillet from the heat and set aside.
Meanwhile, whisk together the olive oil, lemon juice, mustard, garlic, black pepper, red pepper flakes and remaining ¼ teaspoon salt. Set aside.
Once the green beans are done cooking, whisk the olive oil mixture once more, then pour it into the skillet. Pour the almonds back into the skillet, and add about half of the feta, reserving the other half for garnish. Toss to combine, then taste and add more lemon juice or black pepper if necessary.
Transfer the green beans to a serving bowl or platter. Sprinkle the remaining feta and the torn basil leaves on top. Lightly grate some lemon zest on top, and serve promptly.