1. Quarter and seed the tomato. Dice the tomato and transfer to a bowl. Stir in the onion, cilantro, serrano, salt, and half the lime juice. Keep it refrigerated and covered.
2. In a large non-reactive mixing bowl, toss the avocados with remaining lime juice, until evenly coated.
3. Heat the oil in a large saucepan over medium high heat and add onions, garlic and sauté, stirring continuously until onions are translucent, about 5 minutes.
4. Add salt, avocados and stock. Bring to a boil then reduce heat and simmer for 8 to 10 minutes.
5. Remove from heat and puree in a blender. Add more stock if too thick. Refrigerate and serve chilled topped with tomato salsa.