Boil the trimmed green beans in a large pot of salted boiling water for 2 minutes then immediately plunge them in a large bowl of ice water. Heat the butter and oil in a large skillet over medium to medium-high heat. Sauté the shallots for about a minute then add the garlic for about 30 seconds more then add the green beans and a good pinch of salt. Saute for about 3 minutes or until the beans are heated through but still slightly crisp. Turn off the heat and squeeze half a lemon all over the beans. Taste for seasoning and add more salt and/or lemon juice if needed.