Blanch the string beans in a large pot of boiling salted water for 1 1/2 minutes only. Drain immediately and immerse in a bowl of ice water.
Heat the butter and oil in a very large saute pan or large pot and saute the shallots on medium heat for 5 to 10 minutes, tossing occasionally, until lightly browned. Drain the string beans and add to the shallots with the salt and pepper, tossing well. Heat only until the beans are hot.
If you're using regular string beans, blanch them for about 3 minutes, until they're tender.