Cut each eggplant in half lengthwise. Scoop out pulp, leaving a 1/4-inch-thick shell, and reserve pulp. Place eggplant halves, skin sides down, on a baking sheet coated with cooking spray; bake at 350° for 15 minutes.
Chop the reserved eggplant pulp into 1/2-inch pieces. Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add reserved eggplant to pan, and sauté 8 minutes or until eggplant begins to brown. Place eggplant in a bowl. Coat pan with cooking spray. Add onion, and sauté 5 minutes or until tender. Stir in red and green bell peppers and the next 7 ingredients (through squash); sauté 6 minutes. Return eggplant to pan. Stir in 1 1/4 teaspoons salt and black pepper; sauté for 5 minutes. Remove from heat; stir in feta cheese and 1/4 cup panko. Cool slightly. Spoon about 1 cup eggplant mixture into each eggplant shell.
Combine remaining 1 cup panko, remaining 1 teaspoon oil, and remaining 1/4 teaspoon salt. Sprinkle panko mixture evenly over stuffed eggplants. Lightly coat with cooking spray. Bake at 350° for 20 minutes or until thoroughly heated and tops begin to brown. Sprinkle with parsley, if desired.