Heat olive oil in a large skillet or Dutch oven over medium heat. When the oil shimmers, add the beef, onion, and garlic. Cook, stirring, until the meat is no longer pink and the onion is soft, about 7-9 minutes. Drain off the fat, leaving the beef in the pan.
Stir in the pasta shells, broth, tomato sauce, Rotel diced tomatoes with green chiles, taco seasoning, and sugar. Bring to a boil, then reduce the heat to low or medium-low so the liquid is at a gentle simmer. Cook until the pasta is tender, about 8-10 minutes, stirring occasionally so that the pasta cooks evenly and doesn’t stick to the bottom of the pan. Remove from the heat.
Stir in the Velveeta, cheddar, and heavy cream. Stir until the cheese melts. Taste and season with additional salt and pepper, if necessary.
Garnish individual servings with optional toppings.