Step 1

Bring a large pot of heavily salted water to a boil. Meanwhile, dice 1 small yellow onion (about 1 1/2 cups) and mince 2 garlic cloves. Drain 1 (2.25-ounce) can sliced black olives. Finely chop the leaves and tender stems of 5 fresh cilantro sprigs (about 2 tablespoons). Tear 4 ounces American cheese slices into big pieces. Grate 8 ounces sharp cheddar cheese on the large holes of the box grater (about 2 cups); set aside a small handful for garnish.

Step 2

Add 1 pound medium dried shells to the boiling water and cook and according to package directions until al dente, about 9 minutes. Meanwhile, make the sauce.

Step 3

Melt 2 tablespoons unsalted butter in a large high-sided skillet or Dutch oven over medium-high heat. Add 1 pound lean ground beef and cook, breaking it up with a spoon or spatula into bite-sized pieces, for 3 minutes. Add the onion, garlic, 2 tablespoons taco seasoning, and 1 teaspoon kosher salt. Cook, stirring occasionally, until the onion is softened and the beef is browned and cooked through, 4 to 5 minutes. Sprinkle in 1/4 cup all-purpose flour and stir until there is no more visible flour, about 30 seconds.

Step 4

Add 2 cups low-sodium beef broth and scrape up any browned bits from the bottom of the pan with a wooden spoon. Add 1 (14.5-ounce) can diced tomatoes and their juices. Stir to combine and bring to a simmer. Turn off the heat. Add the cheddar cheese, American cheese, and 1/2 cup sour cream, and stir until the cheeses are melted. Taste and season with more kosher salt as needed.

Step 5

When the pasta is ready, drain. Add to the sauce and stir to combine. Garnish with the reserved shredded cheddar cheese, olives, and cilantro.