For the Arepas, place milk, water and 1/4 cup (1/2 stick) of the butter in small saucepan. Cook on medium heat about 5 minutes or just until butter is melted. Remove from heat; cool slightly.
Combine masarepa and sea salt in medium bowl. Pour the milk mixture slowly into masarepa mixture, stirring until liquid is fully absorbed. (Mixture may be slightly lumpy.) Add feta and knead mixture with hands until smooth and cheese is fully incorporated. Divide dough evenly into 12 balls. Flatten each ball into a disk about 4-inches in diameter and 1/2-inch thick.
Keep a bowl of cold water nearby when preparing the arepa dough. If your hands get too sticky while kneading and forming the arepas, wet them in the cold water and continue working dough.
Melt 1 tablespoon of the remaining butter in large cast iron skillet on medium-high heat. Cook arepas in batches about 6 minutes per side or until golden brown and slightly crispy. Adjust heat to medium-low if arepas are browning too quickly. Add additional butter to skillet between batches if needed. Transfer arepas to a platter; cool slightly. (Arepas can be made up to 3 days ahead. Refrigerate in airtight container. When ready to serve, bring to room temperature before reheating in a hot, buttered skillet.)
For the Tabbouleh Salad, bring 1 cup water to boil in small saucepan. Add bulgur and 1/2 teaspoon of the sea salt; return to boil. Cover. Remove from heat; let stand 25 minutes. Fluff bulgur with fork. Cool to room temperature.
Meanwhile, toss lettuce, parsley, feta, tomato, cucumber, olives, green onions, lemon juice, olive oil, mint and crushed red pepper in medium bowl. Add cooled bulgur; toss well to combine. Stir in remaining 1/2 teaspoon sea salt.
For the Yogurt Sauce, whisk yogurt, water and seasonings in small bowl. Refrigerate until ready to serve.
To assemble arepas, slice each arepa crosswise about 2/3 of the way to make a pocket. Spread Yogurt Sauce evenly in each arepa. Spoon about 1/3 cup of the Tabbouleh into each arepa, pressing gently to fill. Top with additional Yogurt Sauce, if desired. Serve immediately.