In Guatemala, leftover stock from pots of boiled beef (cocido) is often transformed into this soup. Here, a simple stock from roasted beef bones approximates that flavor. If you're short on time, use the best premade beef broth you can find.
To prepare broth: Preheat oven to 450°F.
Combine bones, onion, carrots, celery, leek and oil in a large roasting pan. Roast, stirring once or twice, until the bones and vegetables are very brown, about 45 minutes.
Transfer the bones and vegetables to a large stockpot. Add 1 quart water to the roasting pan, scrape up any browned bits, then pour the water into the pot. Add the remaining 3 quarts water, cover and bring to a simmer over high heat. Reduce heat to maintain a simmer and cook, skimming off any foam, for 15 minutes.
Add peppercorns and 1 cup parsley. Continue simmering until any meat falls off the bones and/or the marrow is very soft and easily removed with a spoon, about 2 hours.
Let the broth stand for 30 minutes. Strain through a sieve lined with cheesecloth. Skim fat, if using the broth immediately, or refrigerate overnight and remove solidified fat before using. You should have 10 cups broth.
Meanwhile, prepare tartaritas: Whisk milk and eggs in a large bowl. Add flour and salt and whisk until the batter is smooth. Let rest for 10 minutes. Stir in parsley.
Line a baking sheet with paper towels. Heat 1 tablespoon oil in a small nonstick skillet over medium-high heat. When it's hot, drop batter by the teaspoonful to form crêpes about 1 1/2 inches in diameter. Cook until beginning to turn golden on the bottom, about 30 seconds. Flip and cook until golden, about 20 seconds more. Transfer to the baking sheet. Repeat with the remaining batter and oil in several batches, adjusting the heat if the crêpes are browning too quickly.
To season & finish soup: Combine the broth, salt and pepper in a large pot. Bring to a boil over high heat. Add mint sprigs and cook for 2 minutes. Discard the sprigs. Ladle the soup into bowls and divide the crêpes among them. Garnish with chopped mint, if desired.
To make ahead: Prepare broth (Steps 1-5) and refrigerate for up to 2 days or freeze for up to 4 months.