A 20-minute dinner doesn’t get much better than this classic Irish comfort food. It’s perfect for your St. Paddy’s Day celebration, but it’s plenty delicious enough to make all year long. Shepherd’s Pie is one of those foods you’ll find on any March 17 restaurant menu. You’re practically required to eat it at some point in the month of March. It has become as synonymous with the St. Patrick’s Day celebration as pints of green beer — which is to say, that neither the green-dyed drinks nor this beef-based dinner are exactly Irish. Traditional Shepherd’s Pie uses lamb or mutton (It’s Cottage Pie that uses beef). But here in the U.S., we’ve definitely put our own spin on the holiday, so I think this dish gets a pass, too. I like to cut corners with this recipe and get everything pre-chopped so all I have to do is brown some…