STEP ONECUT chayotes in half; remove seeds and scoop out centers. Place cut side down in microwave safe dish; add 1/2-inch water. Cover with plastic wrap; cook on HIGH (100% power) 8 minutes. (Use caution when removing plastic wrap as there will be steam.) Pierce each chayote with the point of a small knife to check for doneness.
STEP TWOHEAT oven to 400ºF.
STEP THREEHEAT oil in medium skillet. Add onion, garlic and celery; sauté 5 minutes. Add ground pork. Cook until meat crumbles and is cooked through. Add sofrito; season to taste with salt and pepper.
STEP FOURSPOON filling into cooked chayote halves. Sprinkle with grated cheese; bake 10 minutes or until cheese is melted and golden.
STEP ONEHEAT oil in large sauté pan. Sauté carrot and leeks until soft. Stir in tomato sauce; mix well. Cook 5 minutes.
STEP TWOADD chicken stock and cayenne pepper; cook over high heat until liquid is evaporated and sauce is thick, approximately 15 to 20 minutes. Let cool.
STEP THREEPOUR sauce into food processor or blender; blend until smooth. Return to pan; gradually whisk in butter until well blended.
STEP FOURADD chives. Ladle sauce onto plate; place Chayotes Rellenos on top. Serve immediately.