Meanwhile, in a large skillet, heat ¼ cup olive oil over medium-high heat. Add shallots and sauté 3-4 minutes, until soft and translucent. Add the garlic and cook for 20-30 seconds. Season with red pepper flakes, salt + pepper. Add clams, mussels, shrimp, sherry, and lemon zest; cover and cook for 5 minutes (shaking the pan a few times during cooking).