^^^ Exactly this. ^^^

For my inner pair, I wear summer-weight full-finger MTB gloves in size large. Then I wear and extra-large pair of "cold weather" (read: Who cycles below freezing?) windproof, gloves for the outers. This combo gets me down to the mid-teens F. I can still work the STI levers just fine, and if I do need more dexterity, I still have the inner glove on.