Pour the salt into 2 cups lukewarm water. Mix the arepa cornmeal slowly into the salted water until smooth, let stand for 5 minutes. Divide into 4 equal parts and shape the dough into a flat disk.
Heat a griddle or pan over high heat. Place the arepas on a hot griddle or pan and cook until small brown spots appear, 5 to 7 minutes on each side. Finish the arepas in the oven for 15 to 20 minutes.
Insert a butter knife into the arepa edge, cut halfway around the arepa and pop it open for stuffing. Begin with cheese as a base, and then layer the salmon on top. Sprinkle with capers and top with sliced avocado. Repeat for the remaining arepas.