Bring a pot of well-salted water to a boil over medium heat. Set up a bowl of ice water and season it generously with salt.
Add the beans to the boiling water and cook them until they are tender, 4 to 5 minutes. Bite a bean to be sure they are properly cooked. Put them immediately in the bowl of salted ice water to stop the cooking or to "shock" them. When they are cold, remove them from the water and let them dry a little.
Coat a large saute pan with both clarified butter and olive oil. Bring the pan to a medium-high heat, add the shallots and brown. Add the haricot vert, season with salt and saute until all the beans are coated with oil and hot. Serve immediately or later at room temperature.