Pit and dice nectarine. Set aside.
Pat halloumi dry with a paper towel and cut it into thick slices. Grill or dry fry the cheese in a skillet according to the package instructions. When golden brown, transfer the halloumi to a cutting board and cube it. While the cheese is grilling, make the sauce.
Avocado sauce: Pit avocados and mash them in a small bowl. Stir in the sour cream, lime juice, and salt. Taste and adjust seasoning. Cover and set aside.
Briefly heat tortillas – either on the grill or in a dry (no oil) skillet on the stove-top. You can also char the tortillas directly over the gas flames for a few seconds using tongs. To keep them warm, transfer them to a plate or bowl, and cover with a clean dishtowel until all tortillas are warmed.
Assemble tacos: Spread each tortilla with avocado sauce. Top with diced halloumi and nectarine.
Place some mayonnaise into a ziploc bag and cut the tip off to drizzle onto the tacos. Garnish with cilantro and serve with lime wedges.