Remove the bacon skin, if any, and reserve if desired. Cut the bacon into 1 cm cubes. Fry over low heat until all the fat is rendered. Drain on paper towels. Reserve 2 tablespoons of the fat. Heat the reserved bacon fat in a stockpot over medium-low heat. Cook the onions with the salt and pepper until deep brown, 10 to 15 minutes. Add the chicken stock and bacon skin, if using, simmer 15 minutes. Remove the bacon skin. Skim and discard the fat from the top, and bring the broth to a boil. Stir in the fried bacon and tomatoes and return to a boil. Gently stir in the beaten eggs just until cooked and thready, 1 to 2 minutes, being careful not to overblend. Remove from the heat. Ladle into warm bowls and sprinkle with the cheese and serrano chiles. Serve immediately with warm corn tortillas. SOURCE: Mesa Mexicana, Mary Sue Milliken and Susan Feniger with Helena Siegel.