Step 1 Make the aji amarillo purée: Halve the peppers lengthwise, removing the seeds and membranes, then dice the peppers. Purée the peppers in a blender with 1 tablespoon olive oil to form a paste. Remove the paste and set aside (there is no need to wash the blender; it will be used again).
Step 2 Heat 1 tablespoon oil in a small skillet over medium-low heat. Add the onion and cook, stirring often, until softened but not brown, 5 to 7 minutes. Stir in the garlic and continue cooking until it is aromatic, about 1 minute, then remove the pan from heat and transfer the mixture to a bowl to cool.
Step 3Place the onion mixture in the blender, along with the queso fresco, milk and 1 tablespoon plus 1 teaspoon of the aji amarillo purée, and blend until smooth. If the sauce is too thick, gradually pour in a little milk while blending at low speed. Scrape down the sides of the blender. Blend the sauce again until it is smooth. Taste, and add a pinch of salt and more of the pepper purée if desired. This makes a generous cup of salsa. The salsa can be made up to a day ahead and refrigerated, tightly covered; thin with milk before using if desired.
Step 4Reserve the remaining pepper purée for serving separately, and cover and refrigerate if not using right away.