Heat frozen green beans according to package directions. (Cook fresh green beans in large pot of generously salted boiling water until crisp-tender, about 5 minutes. Drain; rinse under cold running water and drain again.).
Melt 1 Tbutter in heavy large skillet over medium-high heat. Add shallots and saute until deep brown and crisp ( I usually just cook until soft and transluscent). (Green beans and shallots can be prepared 2 hours ahead. Let stand at room temperature.).
Melt 1 T butter in heavy large pot over medium-high heat. Whisk in mustard. Add green beans; toss until heated through and evenly coated, about 4 minutes. Season with salt and pepper. Mound beans on platter. Sprinkle with shallots.