Blanch the string beans in a large pot of boiling salted water for just 1 1/2 minutes. Drain immediately and immerse in a large bowl of ice water to stop the cooking. When they are cool, drain and set aside.
Heat the butter and olive oil in a very large saute pan over medium heat and cook the garlic for 1 to 2 minutes, tossing occasionally, until lightly browned. Add the string beans, sprinkle with salt and pepper, and toss together. Reheat the string beans and serve.