Halve peppers lengthwise through the stems, leaving the stems
attached. Remove the seeds and white membrane. Place the peppers cut-side down in a large microwave-safe dish. Add 1/2 inch water, cover and microwave on High until the peppers are just softened, 7 to 9 minutes. Let cool slightly, drain and set aside.
Meanwhile, bring a large saucepan of water to a boil. Add orzo and cook until just tender, 8 to 10 minutes or according to package directions. Drain and rinse with cold water.
Mash chickpeas into a chunky paste with a fork, leaving some whole.
Heat oil in a large nonstick skillet over medium heat. Add onion and cook, stirring, until soft, about 4 minutes.
Add spinach and oregano and cook, stirring, until the spinach is wilted, about 1 minute.
Stir in the orzo, chickpeas, 1/2 cup feta, tomatoes, vinegar and salt; cook until heated though, about 1 minute.
Divide the filling among the pepper halves and sprinkle each pepper with some of the remaining 1/4 cup feta.