Scoop the avocado into a food processor and grate the garlic into paste using a fine grater or finely chop garlic and mash into paste with a little salt. Add to avocado along with the juice of a large ripe lemon, about 3-4 tablespoons, and the sour cream or crema. Process until very smooth and season with salt to your taste. Transfer to a bowl and cover with plastic. Chill until ready to serve.