1 lb. Yukon gold potatoes, peeled and cut into 2-inch pieces
2 tbsp. unsalted butter, at room temperature
2 tbsp. olive oil
1 large onion, chopped
2 medium carrots (about 8 oz), peeled and cut into 1⁄4-inch pieces
1 1/4 lb. lean (at least 90%) ground beef
3 tbsp. tomato paste
1/4 tsp. ground cinnamon
1/2 c. beer (lager or stout)
1/2 tsp. beef bouillon
1/2 c. flat-leaf parsley, roughly chopped
Fresh thyme, for topping
Place potatoes in medium saucepan, cover with cold water, and bring to a boil. Add 2 teaspoon salt, reduce heat, and simmer until tender, 12 to 15 minutes. Reserve 1⁄2 cup cooking liquid, drain potatoes, and return to pot. Mash with butter, adding some reserved cooking liquid (2 tablespoons at a time) to reach a smooth mash.
While potatoes are cooking, add oil and onions to 9- to 10-inch cast-iron skillet and cook, covered, on medium for 3 minutes.
Add carrots and cook, covered, stirring occasionally, until vegetables are just tender, 5 to 6 minutes. Add beef and cook, breaking up with spoon into small pieces, until no longer pink, 3 to 5 minutes.
Stir in tomato paste, cinnamon, and 1⁄2 teaspoon each salt and pepper and cook 1 minute. Stir in beer, 1/2 cup water, then bouillon, and simmer until mixture slightly thickens, about 3 minutes. Fold in parsley.
Heat broiler. Spread potatoes over beef and top with thyme if desired. Broil until golden brown, 2 to 4 minutes.