Crush the tomatoes by hand and add them to the pot along with their juices. Add the cumin, garlic, salt, pepper, corn, and cheese and return to a boil.
In a small bowl, dissolve the cornstarch in 1 cup of cool water. Slowly add the cornstarch slurry to the boiling soup, stirring constantly. Simmer for 10 minutes.
While the soup simmers, sauté the ground beef with the chopped onion in a skillet over medium-high heat. Drain off any grease.
Remove the soup from the heat, add the sautéed beef and onions to the pot, and stir.
Serve with sour cream, shredded cheddar cheese, and tortilla chips.