Whisk chickpea flour, salt, and 5 1/2 cups water together in a bowl. Cover and set aside at least 4 hours and up to overnight.
Preheat oven to 500°. Whisk 3/4 cup oil into chickpea batter.
Heat two 10-in. cast-iron skillets or ovenproof frying pans in oven, 5 minutes. Swirl about 1 tbsp. oil into each, then pour enough batter into pans to come about 1/3 in. up sides. Bake 20 minutes, or until edges are crisp and golden brown and centers are lightly browned on top.
Meanwhile, peel and seed peppers, then cut into wide strips.
Lift each farinata out of its pan to a plate or serving board and top with either red peppers or mortadella. Cut each into 8 wedges. Serve immediately. Repeat with remaining batter and ingredients to make 1 more farinata.
*Find chickpea flour at well-stocked grocery stores. The Italian kind, which tends to be fresher and tastier, is available at Whole Foods Market. If you can't find oil-packed peppers, broil 2 large red bell peppers, turning often, until they're blackened all over; then peel and seed them, cut into wide strips, and marinate in olive oil for at least a day.
Note: Nutritional analysis is per wedge.