Blanch the string beans in a large pot of boiling salted water for 3 minutes until they are crisp and tender. Drain immediately and immerse in a bowl of ice water. Let sit while sauteing the shallots.
Heat the butter and oil in a very large saute pan (12-inch diameter) and sauté the shallots on medium heat for 5 to 10 minutes, tossing occasionally, until they just begin to show color and are translucent.
Drain the string beans and add to the shallots with 1/2 teaspoon salt and pepper, tossing well. Heat only until the beans are warmed through.
Notes
If using haricot verts, blanch for only about 1 1/2 to 2 minutes before transferring to ice water.