Bring a large pot of lightly salted water to a boil. Add green beans, and cook uncovered until just tender, 3 to 4 minutes. Meanwhile, melt the butter with the olive oil in a large skillet over medium-high heat. When the beans are almost tender, drain and add to the skillet. Continue cooking the green beans in the butter until they are tender, about 2 minutes more.
Season the beans with salt, pepper, and lemon juice. Place onto a warmed serving dish, and sprinkle with crumbled feta cheese.
