Using a knife, split peppers in half lengthwise. Remove seeds and set aside.
Add tomatoes, red onion, minced parsley, garlic, lemon juice, salt, and pepper to a medium bowl. Gently mix until blended.
Fill peppers with 2 teaspoons of mixture.
Top with a sprinkle of feta cheese and parsley leaf garnish.
Drizzle olive oil and balsamic vinegar on top.
Serve at room temperature or slightly chilled.