Add minced Serrano/jalapeno/chili, chopped cilantro and lime juice to a high speed blender and mix until smooth.
Deseed and cut the tomatoes into small pieces, chop the red onion.
Combine the tomatoes and red onion (leave about 1/2-1 tablespoon for topping) with the lime chili mixture in a bowl. Season with salt and check the taste. Place in the fridge while you prepare the guacamole (preferably at least 10 minutes). Remove the excess water from the bowl before you assemble the tartar to prevent it from turning out too wet.
Next add the minced red chili, chopped yellow onion, cilantro and lime to the high speed blender and mix until smooth.
Assembly the tartare by placing half of the avocado mixture at the bottom of a plate using a round mold, making sure it gets a circulare form and stays in place. Gently push the avocado down to make sure it sits. Place the rest of the avocado in a seperate mold on a seperate plate.
Next divide the Pico De Gallo between the two molds, pressing down gently. Top with the red onion that you have set aside and serve with nachos.