Finely chop the Tomato (1 kilogram), Serrano Chili (8), White Onion (1) and Garlic (1 clove). Mix well in a bowl and season with Coarse Salt (1 tablespoon) and Freshly Ground Black Pepper (1 tablespoon). Cover and place in the fridge for 24 hours to marinate.
The next day place the salsa in a saucepot over low heat and cook until it starts to boil. Add the Fresh Cilantro (3/4 teaspoon) and lower the heat even further. 10 minutes later remove the lid and raise the heat, once boiling add the water mixed with Corn Starch (1 tablespoon) and once it thickens turn off the heat.
Serve warm or chill for later use. Works great with steaks, quesadillas, tacos, etc. In this recipe, I served it on top of a warm quesadilla!