Make the sumac yogurt: Press garlic though a garlic press onto a cutting board and add a pinch of salt. With the side of a chef’s knife, push back and forth on garlic until a paste forms. (A couple of strokes usually does the trick.)
In a medium bowl, combine garlic paste, sumac, oil, and yogurt and mix well. Add salt to taste.
Roast the carrots: Preheat oven to 375 degrees. Peel carrots and toss with olive oil, salt, and pepper. Roast until slightly al dente, about 30 to 40 minutes. Cool to room temperature.
Spoon yogurt onto a plate and place carrots on top. Serve with extra yogurt.